A response to life’s inevitabilities: introducing the Aldgate Junction

In a previous post, I described the mathematical inevitability of decreased frequencies on branchlines. But I also alluded to a particular feature that certain systems have that allows them to bypass this particular pitfall. And that brings us to today’s post. 

Introduction

Let’s take a simple trunk-and-branch system, which sees 10 tph six-min headways on the core and 5 tph twelve-min headways on the branches:

If those branches are out in the suburbs, those lower frequencies may be justifiable. But what if your system needs to branch within the core?

Introducing the Aldgate Junction:

(“Aldgate Junction” is a term I’ve coined to describe this kind of infrastructure and service pattern; there are other related terms, but I think the way I’ll be using this term is a useful addition to our vocabulary.)

An Aldgate Junction enables a second layer of service to travel in one branch and out the other, without touching the core and (usually) without a reverse move. Sometimes this can be created via a relatively minor infrastructure change, such as adding a third leg to create a wye junction, but sometimes a more significant capital project is required.

The original Aldgate Junction

The original “Aldgate Junction” is actually a trio of junctions and is located at the interchange of the District, Metropolitan, Circle, and Hammersmith & City lines:

(Image modified from Wikipedia’s version.)

You can see the layout of the three junctions and their names on this track map from CartoMetro.

The Aldgate Junction concept is deployed here to create a triangle of one-seat rides:

  • Liverpool St to Aldgate East via the H&C
  • Aldgate East to Tower Hill via the District Line
  • Tower Hill to Liverpool St via the Circle Line

Modifying my original diagram above to match the colors allows us to see how the Tube’s topology maps on to the generic model I’m discussing here:

The downside of the original Aldgate Junction in London is that it’s a level crossing, meaning trains block each other as they pass through, which creates a bottleneck. Some Aldgate Junctions utilize flying interchanges to avoid conflicts and increase capacity. 

Real-life examples of Aldgate Junctions

There are lots of ways to deploy an Aldgate Junction. It can provide distributed service to decongest central transfer points, or can be used to serve pluricentric regions with multiple “downtowns”. It can also be used to increase frequencies within the corridors of the branchlines, which is valuable for local trips as well as cases where the branch line is unusually long. 

Northern California

In my previous post, I mentioned that BART – a rather exceptional rapid transit system due to its enormous reach and mid-low frequencies on each branchline – uses an Aldgate Junction to increase frequencies on its East Bay services. 

BART’s Orange Line (also still sometimes called the verbose “Richmond-Berryessa/North San Jose Line”) plays the role of the “branch-to-branch bypass service”. For example: without the Orange Line, stations like Fremont would only see 15-min headways; Fremont is about 30 miles away from San Francisco, so 15-min headways might be appropriate for that service. But Fremont is only 15 miles away from San Jose, and there is a steady level of density running all the way up the East Bay – there are plenty of local journeys that sit squarely in “Rapid Transit Land”, and 15-min headways underserve that. 

Providing the Orange Line overlay raises headways to less than 10 minutes throughout the entire West Bay, without impacting capacity in the San Francisco core. Key to this is the Oakland Wye, a three-way flying junction to enable crossovers without interference. This is an example where an Aldgate Junction required significant infrastructure.

There are numerous other systems that use variants of the Aldgate Junction. 

The VTA Light Rail in San Jose has one between Tasman, Champion, and Baypointe. This is a simple flat junction in a four-way intersection.

Southern California

Moving further down the coast, we find that Los Angeles’ Metrolink commuter rail system actually has two Aldgate Junctions, of different varieties.

First, we see that the Orange County, Inland Empire-Orange County, and 91/Perris Valley lines form an Aldgate Junction in Anaheim. This creates a three-way link between Orange County, LA County, and Riverside County.  

But there is a second, more abstract Aldgate Junction, formed by the Orange County, Inland Empire-Orange County, and San Bernardino Line, between LA County, San Bernardino County, and Orange County. 

In both cases, these exemplify the use of Aldgate Junctions to serve distributed areas of density. Los Angeles is the core of the system, but Riverside and San Bernardino are major urban centers in their own right, and the use of the Aldgate Junction model creates two sub-networks based in Riverside and in San Bernardino. 

The “Riverside Network”
The “San Bernardino Network”

These examples, it should be noted, are cases where an Aldgate Junction is not functioning to increase frequencies; Metrolink’s frequencies are pretty poor, usually only about 1 tph at peak, and little-midday service. In this case the focus is on providing one-seat-rides across a decentralized network, enabling transit journeys to non-Los Angeles destinations that otherwise simply wouldn’t be possible.

New York & New Jersey

Finally, the reigning North American monarch of the Aldgate Junction is New York City.

The New York City Subway has several physical Aldgate Junctions, a notably consistent design feature of the old IND network, highlighted below. 

The PATH system is built around a pair of Aldgate Junctions (forming an impressive set of underground flying junctions):

And the Hudson-Bergen Light Rail offers a classic example of a simple Aldgate Junction near Hoboken Terminal:

PATH and the HBLR offer an additional lesson about the operational realities of Aldgate Junctions; the maps above represent the daytime versions of those networks. On weekends (and in PATH’s case, at night), the Aldgate Junctions functionally disappear.

All journeys remain possible, but some require transfers. 

PATH offloads its HOB-WTC journeys into a two-seat journey with transfer at Grove St; meanwhile, its two routes out of 33rd St are combined into one, with a reverse-move at Hoboken as part of the revenue service pattern. (Functionally, Hoboken is serving as a transfer point in the same way as Grove St; PATH just does the transfer for the passengers by throwing the switches rather than swapping out trains.) 

HBLR maintains weekend service to Hoboken, but riders from the north must transfer at Pavonia/Newport and backtrack. 

Summary

Aldgate Junctions provide a way to distribute service across a larger area and offer a way to increase service levels on branches without being constrained by trunk line capacity. Their physical form and implementation vary widely, from level wye junctions to complicated flying interchanges to dispersed routes spread across miles. These forms defer in capacity, reliability, and cost.

Despite these differences, Aldgate Junctions are united by consistent contributions to network topologies, and are a valuable tool when designing (and future-proofing) complex systems. 

Inevitabilities in Life: Death, taxes, and decreased frequencies on branches

In my previous post about branches, I briefly discussed the rapid decrease in frequencies as you add more branches to a trunk line. You might remember a diagram that I showed: 

A service level diagram, where the trunk has 15 tph, and then 5 tph branch off, leaving only 10 tph to the next station, after which the line splits into two branches of 5 tph each

In this diagram, the trunkline sees high-frequency headways of 4 minutes (which is better than many subway lines in North America). With such a high frequency, it’s easy to think that there’d be enough trains to serve a bunch of branches.

But as you can see, four-minute headways equals 15 trains per hour. If you have three branches, that means each branch gets 5 trains per hour – which yields 12 minute headways. If these branches are out in suburbia, 12 minute headways might be appropriate, but you’ve nearly reached the limit. If you were to add a fourth branch, each branch would see less than 4 trains per hour, at 16 minute headways, at which point you really no longer have a claim to “frequent service”.

In my last post, I recommended, as a rule of thumb, no more than two branches per line. However, I didn’t explain why. It comes down to a combination of typical throughput capacities, and mathematical inevitabilities. 

Divvying up trains-per-hour among multiple branches

Consider this chart:

I will post a text version of this table in the next few days!

On the left side, we have a list of potential trunkline capacities, measured in trains per hour. These indicate how many trains in each direction you can squeeze through your trunkline in an hour. (Don’t worry about converting these numbers to headways – I’ll get to that below.) 

For some perspective (most numbers pre-covid): 

  • BART ran 16 tph through its core section from Daly City to West Oakland
  • CTA ran between 12 and 20 tph on its Red and Blue Lines during peak
  • WMATA ran 15 tph on its Red Line during peak
  • MBTA ran 15 tph on its Red Line during peak
  • London Underground’s throughputs vary widely from line to line, with some lines seeing over 30 trains per hour, following major infrastructure and modernization improvements
  • Beijing’s subway runs between 30 and 35 tph on several of its routes
  • Shanghai’s subway runs between 15 and 32 tph on most of its inner routes

All of which is to say, the top few rows represent trunkline capacities that require major investment in transportation infrastructure.

To the right of those trunkline capacities are the number of trains available to each branch, depending on how many branches you have. So, for example, in the first row, a 40 tph trunk will provide 20 tph to two branches, 13.3 tph to three branches, 10 tph to four branches, and so on. 

As mentioned before: you’ll notice that, as you move from left to right across the chart, the numbers in each row drop dramatically. In fact, the decrease is literally exponential; you can describe the chart above using

y = n-x

where n is the capacity of your trunkline in tph, x is the number of branches, and y is the resulting tph per branch.

Decreasing frequencies due to decreasing tph

It’s helpful to start this discussion using tph as a measure, because it’s easier to recognize the patterns in the numbers’ decrease. However, once we convert those tph into headways, it’ll become that much clearer why branching quickly leads to decreased frequencies.

I will post a text version of this table in the next few days!

I’ve added some (opinionated) color-coding, meant to suggest the various “levels” of service these different frequencies provide. 

  • The bright green (every 5 minutes or better) represent the highest tier of frequent service an agency might provide. “Turn up and go.”
  • The pale green (every 5-10 minutes) are still comfortably in the realm of rapid transit, but are probably better suited to off-peak periods and lower-ridership networks. “Turn up and wait a couple minutes and go.”
  • The yellow (every 10-15 minutes) are the lowest tier of what could be considered “turn-up-and-go,” describing services where riders don’t need to check a schedule when planning journeys. Call this tier “turn up and wait.”
    • This tier should be approached cautiously, with careful attention paid to the specific corridor where these frequencies would be deployed.
    • (Sadly, consideration should also be given to reliability; if a bus is scheduled to show up every 13 minutes, but one run gets dropped, suddenly you have folks waiting nearly a half-hour for their bus.)
  • The subsequent tiers are rarely going to be considered “frequent service”; some could be “salvaged” by adhering to a strict clockfacing schedule: a route that reliably comes exactly :13 minutes and :43 minutes past the hour can be a useful service that isn’t “turn-up-and-go”, but doesn’t require consulting a schedule either. These tiers break down something like this:
    • Orange = “plan when to leave, but journey whenever”
    • Light grey = “plan when to journey”
    • Dark grey = “schedule around the schedule” 

As you can see, our frequencies drop through the tiers I’ve described above quite quickly. Once you hit the yellow tier, you’re teetering on the edge of frequent service, and once you hit the orange and beyond, you’re definitely over the edge.

Different regions will have different definitions of “high frequency”. For example, it’s pretty rare to wait more than 4 minutes for a tube train in central London; a six-minute headway would be considered sub-par. In Boston, we are sadly accustomed to six-minute peak headways on the Orange Line, while Baltimore’s subway sees peak service every 8 minutes. 

It’s worth highlighting that reaching 90-second headways at 40 tph on heavy rail is exceptionally difficult. If you have multiple tracks in the same direction, it becomes more manageable, but a standard two-track subway is nearly impossible to operate at 40 tph per direction, outside of the world’s most advanced subway systems. And notice – even the top examples I listed above, such as Beijing or London, you’re still looking at that second row as your baseline, at around 30 tph. Even in those systems, having more than two branches knocks each branch down into a lower “tier”, meaning it’s not suitable to do within the urban core. 

If your crayon map requires shoveling 40 trains per hour through a trunk line, then probably it’s worth trying to plan a second trunk line!

Gaming out examples, and dealing with the unideal real world

Many of the North American systems I mentioned above see 4 minute headways on their core. If we go to that row, we can see two branches gives us 8 min (good so far), three branches gives us 12 min (borderline), and four branches gives us 16 min (no good). 

Some North American systems see base headways of 7 or 8 minutes. In that case, our drop-off happens even faster: 2 branches becomes borderline, and 3 branches sinks us with headways longer than 20 minutes.

On the London and Chinese systems mentioned above, trunklines see headways around 2 minutes or better. In theory, those trunklines could accommodate five or six branches; but if you drop the core headways by just one minute, suddenly you can only accommodate three or four branches at similar frequencies. 

And that “in theory” caveat is what’s really going to get us. Even if you can squeeze 30 tph through your trunkline to get 2-min headways, if you are feeding that trunk from five branches, that’s five times as many opportunities for delays and disruptions. We just saw above that the difference between a 2-min headway and a 3-min headway is worth two whole branches of throughput. That’s a very thin margin for error – meaning that you need reliability to be extremely high, or else the whole system will unravel, with cascading delays across your network.

The exceptions that prove the rule: North American legacy subway-streetcar networks

There are only two networks that I’m aware of which see sub-10 minute headways on four or more branches, feeding into a trunkline handling 40 tph: the MBTA’s Green Line and SEPTA’s Subway-Surface Lines.

This level of throughput is achieved (see caveat below) mostly because they are light rail lines rather than heavy rail. This means shorter trains which can start and stop faster, and therefore can be run closer together (albeit at lower speeds). The MBTA allows multiple Green Line trainsets to enter certain stations simultaneously, and SEPTA actually treats a couple of its subway stations as “request stops”, with trolleys rolling through non-stop if no one signals to board or alight. 

(I haven’t done the math on this, but it would be worth someone calculating the actual capacity of those two systems compared to heavy rail equivalents. It is true that the TPH levels are higher, but since the trains are shorter, I don’t know that you actually end up carrying more passengers.)

The reality, sadly, is that both of these networks are infamous for their reliability issues. (Full disclosure: I’m much more familiar with the MBTA than SEPTA, but I believe most of the T’s problems also exist in Philadelphia.) Delays are common, both on the branches and then resultantly in the core, potentially resulting in less than forty actual trains per hour through the core. 

Having four or five branches is somewhat workable on these two systems due to their unusual characteristics. Replicating that success elsewhere would likely require replicating those characteristics as well.

The other exceptions that prove the rule: hybrid rapid transit/commuter rail systems

BART’s core trunkline from Daly City to West Oakland feeds out into four eastern branches (Richmond, Antioch, Dublin/Pleasanton, Berryessa/North San Jose). Likewise, the London Undergroud’s Metropolitan Line runs to four termini in the northwestern suburbs (Uxbridge, Amersham, Chesham, Watford). 

Both networks accomplish this by running service to distant suburbs that is more like high-frequency commuter rail than traditional rapid transit service. Each branch on the BART runs every fifteen minutes, which isn’t super unreasonable given that most of those branches travel over thirty miles from downtown San Francisco. (BART is also a pluricentric network, which makes the ridership patterns a bit different than, for example, Chicago’s.)

The Met in London is a bit more complicated, but the concept is similar. Similar to New York, and unlike BART, the Metropolitan is quad-tracked for certain stretches, and therefore runs both local services and express services. During peak hours: 

  • Uxbridge sees 10 tph (6 min) spread across local and express services, with 4 of those trains short-turning at Baker Street
  • Amersham sees 4 tph (15 min), half of which are express
  • Chesham sees 2 tph (30 min)
  • Watford sees 8 tph (7.5 min), again with some short-turns and some expresses

A couple of further notes for context:

  • Almost all of the Uxbridge branch is also served by Piccadilly trains – 12 tph at peak – providing a robust 22 tph, which creates headways under 3 minutes
  • The Amersham and Chesham services run together until Chalfont & Latimer, before splitting off and going one stop each to their terminus, meaning most of the branch in fact sees 6 tph; these services are also supplemented by at least 2 tph that run on Chiltern Railways to Amersham and beyond. 
  • Chesham is also the most distant Underground station – 25 miles from Charing Cross in central London, and technically outside of Greater London – well into territory where 30-minute headways might be reasonable
  • The town of Watford is served by three other stations; the largest of these is Watford Junction, which is less than a mile from the Met station, and sees 4 Overground trains and about 5 London Northwestern Railways trains toward London per hour

So, the Met has a few mitigating factors:

  • Its branches are supplemented by additional services
  • Despite four possible termini, really it just has three branches, one of which splits at the very end
  • The only stations that see 15-minute-or-worse headways are distant suburbs, over 20 miles from the core of London

Like SEPTA and the MBTA, both BART and the Metropolitan also have unusual characteristics that enable them to get away with breaking my “two branches max” rule of thumb.

However, both BART and the Met also have an additional special feature that allow them to have their cake and eat it too… to be continued in the next post

Branches are not your friend

When I was a kid, I wanted to send branches everywhere. As a child, it was an easy concept to understand — I knew that branches came (like the Braintree or Riverside branches) and that branches went away (like Watertown), and if the Green Line used to have five branches, maybe it could have six or seven or eight! And if the Green Line could have eight branches, why couldn’t the Orange Line? 

Obviously, with age has come a modicum of learned wisdom; I stopped drawing branches everywhere and tried to focus on only sending branches where train service actually was reasonable. But there is more to the picture than that.

It is tempting to think of subway branches like roads. If you build a new road, it means that more places are connected. Providence and Hartford don’t have an interstate highway between them — you have to drive north or south first and pick up Route 84 or Route 95, and then head west. It takes longer than it would if there were an interstate directly between them, which is why you will occasionally hear proposals for one. Two disconnected places ought to have a connection between them.

Rail systems don’t work like that most of the time (and actually road systems don’t either, but that’s a different topic).

There actually is a surprising amount to discuss on this topic! Here are some rules of thumb:

Two branches max

For most systems, do not give an HRT/subway line more than 2 branches. A light rail line, or a commuter/regional rail line, can probably take more, but keep in mind the other rules of thumb below.

One evening I decided to jump down this rabbit hole and did a cursory review of pretty much every heavy rail subway system in the world. Almost none had HRT lines with more than 2 branches. Most of the exceptions were themselves exceptional systems, such as BART or London’s Metropolitan Line, both “subway” lines that act more like commuter rail lines, especially along their branches.

Consider frequencies

Branches have low frequencies, trunks have high frequencies. Trunks should only branch once they are far enough from the urban core that lower frequencies will be feasible. 

Most transit systems in North America have a “full-build reach” (even if hypothetical) of a 10 mile radius from the center. Your trunk shouldn’t branch too close to downtown; every city will be different, but in general you’ll want your trunk to branch no closer than 5 miles to the core. If you need to branch closer to the core, you should try to create a second trunk line.  

Remember that branching frequencies are almost always a hard-and-fast math problem: If your trunk line can take x trains per hour max, then two branches can each feed no more than 50% of x. That is often going to be enough to knock a “high-frequency” trunk service into a “mid-frequency tier” on the branches. As such, it’s worth conceptualizing a branch as something less than a “full” rapid transit line.

In the diagram below, note how the trunk line has a high frequency of 4 minute headways via 15 trains per hour, but that splitting it up into three branches very quickly drops frequencies to 12 minutes on each branch.

A service level diagram, where the trunk has 15 tph, and then 5 tph branch off, leaving only 10 tph to the next station, after which the line splits into two branches of 5 tph each

Read more about decreasing branch frequencies in my follow-up post on this topic.

Trunk line capacity should be your starting point

It’s important to recognize that branches are (usually) a game of subtraction, not addition. Your trunk line provides the pool of trains that you can send out to multiple branches. Unless you know that the trunk line is under capacity, you should assume that they are shoveling through as many trains as they possibly can, and can’t add more. So you need to think about redirecting the existing pool of trains, which is why this is a game of subtraction and not addition. 

Note that I said this is usually a subtraction game – some newer/younger rail systems will not be at capacity. When that is the case, a branch proposal can work in your favor: proposing the addition of a second mid-capacity branch can combine with the existing mid-capacity service to create high-capacity service in the core (where presumably demand will be higher). Especially in newer metro systems, there may not be sufficient existing demand to justify the capital expenses of expanding the fleet to increase service in the core; adding a branch can broaden your revenue base, and enable the purchase of enough vehicles to run high-frequencies in the core. (Sometimes – these calculations are always going to be complicated.)

Look for short-turn services (and over-capacity extremities)

Transit agencies are incentivized not to run extraneous services. The further out from the core, the lower the demand for service (very much theoretically, but I digress). One solution agencies use are short-turn services, in which some fraction of vehicles do not run the full-length of the route. Sometimes this is done to reduce the number of near-empty vehicles running on the outskirts of the line, and sometimes this is done in order to increase the reliability of service on the inner sections of the line (and sometimes both). Short-turns are very common on bus routes, but do have their role in rail transit as well. 

If your trunkline has an existing short-turn service, that is a very strong jumping-off point for a new branch: it points to a confirmed high-frequency trunk/mid-frequency branch demand model, and it doesn’t require additional capacity to be freed up in your trunk, since you aren’t trying to funnel a greater number of trains per hour through. (You will still need to expand your fleet size to maintain existing frequencies, although you may be able to adjust frequencies elsewhere in the network to account for the longer trips your short-turn services will now be taking on the branch).

A simple route diagram, from "Downtown" to "Beltway" to "Suburbia", before and after. Before, half of all trains terminate at Beltway (indicated by the line thinning to halfwidth). After, half of all trains go to Suburbia, and the other half divert after Beltway to East Suburbia, on a new branch.

But be careful – this technique works well if the short-turn is intended to avoid excess capacity on the extremities of the existing service. If the short-turn is intended instead to increase reliability within the core, then extending those short-turns out onto a new branch may jeopardize that reliability. And make sure to take note of the pitfalls of junctions, below.

Be cautious with reverse branching

Be cautious with reverse-branching, which is when a transit service splits into multiple branches going in to the city, rather than going out. (SEPTA’s Broad-Ridge Spur is a good example of this on-paper. In practice, it’s actually a little more complicated, but that’s a post for another time.)

A SEPTA system map with the Broad-Ridge Spur highlighted

http://www.septa.org/maps/system/

Recall what we went through above: branches are lower frequency than trunks. Reverse-branching into an urban core thus means you are reducing frequency precisely in the area you need it most. 

Note that I say “be cautious,” not “avoid.” I think that one of the major values of crayon maps is that they sometimes value creativity over feasibility. Reverse-branching is usually more creative than it is feasible, but sometimes it sparks good follow-up ideas. So, I wouldn’t forbid it as a hard-and-fast rule, but it’s good to be aware of its drawbacks.

Junctions are complicated

Junctions where branches come together will always have one of two drawbacks, so be careful where you put them:

If a level junction is used, then you will have capacity limits and sometimes delays, as trains need to wait for each other to cross, and may need to leave a whole signal block free (i.e. the train may not be able to wait right at the junction like a car at an intersection, but may need to be hundreds or thousands of feet away).

A flying junction avoids those problems and should be the standard for any new rapid transit junction. However, flying junctions are much more expensive, and also take up more space — horizontally and vertically — than level junctions. So, as with all things, it’s a trade-off between cost and quality of service. If you want your proposal to be taken seriously, make sure your proposal is ambitious enough to merit building the flying junction. 

In conclusion, branches are not your friend

Branches are not the friend of the crayon-mapper. They look great on paper but require a lot of careful planning to be done well; when done poorly, they can be actively detrimental to the individual branches and the system overall. If you really want to use them on a subway network:

  1. No more than two branches
  2. Aim to branch at least 5 miles from the core
  3. Look for segments where there already is reduced service (short-turns) or reduced demand for service (lower density)
  4. If both branches demand full-frequency service, then both branches warrant their own trunklines through the core: i.e. they shouldn’t be branches, but should be separate lines

Post-script on LRT, BRT and mainline rail

All of the dynamics I describe here hold true on other modes of service. However, the cost-benefit calculations work out a little differently because there are different standards for these modes.

LRT

For the most part, LRT is going to have the same struggles as an HRT subway would. The main differences are all knock-on effects of light rail’s shorter and more nimble rolling stock. Shorter trains makes it easier to “fudge” capacity in a trunk line, with multiple trains stopping in a single station at once, and shorter signal blocks allowing trains to run closer together. The shorter signal blocks also mean that flat junctions – while still disruptive – can be mildly less problematic. 

LRT also – by virtue of its lower capacity – is sometimes a better fit for serving lower-density regions, which can sometimes mean that the frequency cost of more than two branches is more manageable, especially if combined with a higher frequency on the trunk than you could achieve with larger trains.

Boston and Philadelphia both run a scheduled 40 light rail trains per hour through their tunnels (Philly apparently swings even more than that), with four and five branches respectively. San Francisco runs at a lower frequency (I think) in the Muni Metro subway, also with five branches. All three of these systems are plagued with reliability and other issues. (Muni recently tried to redirect two of its branches out of the subway, but now has returned them to the tunnel). That isn’t to say that these systems can’t be improved, nor am I denying benefits to their approaches. But it’s probably best to think of these as the exceptions which prove the rules.

BRT

BRT comes in lots of flavors, some of which are more amenable to branches than others. Christof Spieler gave an excellent presentation at TransitCon 2022 on the varieties of BRT, and discussed some of the pros and cons of branching on each. In short, I’d say that the more you want your BRT line to act like a subway line, the more you should follow the same rules about branching. 

Mainline rail

Mainline rail faces all of the same dynamics I outlined above. The major difference is that the costs of branching don’t always end up being “disqualifying.” If you take a 20tph rapid transit line and give it 10 branches, each branch will get 30 minute headways, which no longer is “rapid transit.” But if you give a 20tph commuter rail line 10 branches, the 30-minute headways on each branch may be perfectly reasonable. The same dynamic is at play — it’s just that the requirements are different and therefore the costs are more acceptable. 

On the other hand, some dynamics remain equally if not more problematic. Junctions, for example, will still be disruptive if built flat and costly if built flying. And reverse branching’s impact may be felt even more acutely: many American commuter rail lines run once per hour; split those between two downtown terminals – we’ll call them “Northtown” and “Southtown” –, and commuters now have to wait two hours between trains, which reduces flexibility for riders and may have an overall chilling effect on ridership on both branches. 

Consider this fictional system below:

Inspired by the NJ Transit system, four routes of 2 tph each branch out to Burlington (green), Louisburg (red), Toledo (blue), and Millville (purple). Those last two merge at Arroyo, proceding north to Belleview where they are joined by Red, all three of which proceed to Newtown where they are joined by Green. Between Newtown and Riveredge are 8 tph. East of Riveredge, each line splits into two half-width thinner lines, one each to Northtown and to Southtown

At first glance, it looks like a relatively robust commuter rail network. Each suburb sees 30-minute headways, and that segment between Newtown and Riveredge sees rapid transit-frequencies of 7.5 minutes. And every community sees direct service to both Northtown and Southtown.

But most commuters don’t care about going to two different workplaces – their destination is the same everyday. Let’s look at what the network looks like for a Northtown commuter:

A modified version of the map above, showing only services that go to Northtown. Each line is now half-width all the way through.

This paints a significantly different picture. Most suburbs are reduced to hourly service, and the turn-up-and-go frequencies at Newtown are gone. 

(To be fair, in this example, most American cities would be thrilled to see commuter rail service at this level – hourly commuter rail is nothing to sneeze at. But I’m keeping the numbers simple to keep the math simple; if most cities start with hourly commuter rail, then reverse branching drops the effective frequency of each branch down to every two hours, which is pretty rough. You get the idea.)

Instead of a robust trunk-and-branch network, reverse-branching reduces this network into two significantly diluted networks operating in parallel. 

The same map as above, but for Southtown, shown at 50% size to compare to the Northtown map at right.
The same Northtown map as above, shown at 50% size to compare to the Southtown map at right.

How to Draw Transit Crayon Maps (part 1 of many)

There are many ways to draw crayon maps – many more than could fit in one post. In this post I will talk about the software that I use to create maps for this blog.

Software

Really we should be starting with something more fundamental like “pen-and-paper” or “the ideas”. But, assuming you’ve got some of the basics in mind, one of the big questions next is the software. (Although, don’t underestimate the effectiveness of a good hand-drawn diagram that is scanned/photographed and uploaded! No need for software if you feel like going old-school.)

It’s worth distinguishing two kinds of goals one can have with transit maps and diagrams. First, one can focus on elegant design solutions (especially for existing systems); TransitMap.net is a great blog that focuses on transit map/diagram design. Second, one can focus on conveying detail, particularly of proposed systems, with less focus on “looking good”. While these two obviously overlap in significant measure (elegant design also usually is particularly good at conveying detail), my focus is usually on the latter more than the former.

Put another way: I don’t worry too much about my diagrams looking pretty. In my case, this is somewhat self-serving, because I don’t have the know-how to make good-looking diagrams at a pro or even semi-pro level. But put more positively: this also means that you don’t need to worry about getting your diagrams to look good either! Don’t let that be a barrier to getting started!

Okay, but actually, what software should I use?

Okay, sorry for getting distracted. The short answer: start with what you have, and then try some of the ones I suggest here. 

Microsoft Paint or Paintbrush (for Mac)

I can guarantee you that any designer who happens to read this post will physically grimace at this suggestion, so I do want to be clear that both of these programs are extremely limited; if you are going to use them, keep things simple: straight lines, 45-degree angles, simple text. 

That being said, especially when you just want to sketch something out, Paint is a very useful tool, precisely because of its simplicity. With some effort and care, you can indeed use it to make clear, straightforward diagrams. 

For example, this is something I threw together in MS Paint just now, illustrating a possible commuter rail network for Rhode Island, with the existing MBTA Commuter Rail line, a new RI Commuter Rail service from Worcester to Westerly, and an extra layer of local trains running between Pawtucket and TF Green:

A simple diagram showing a commuter rail network for Rhode Island. In addition to the MBTA's Providence Line (running to TF Green Airport, with the under-construction station at Pawtucket now open), a light blue line runs from Worcester to Westerly, with intermediate stops at Woonsocket, Manville/295, Pawtucket, Providence, Olneyville, Cranston, TF Green Airport, East Greenwich, Wickford Junction, and Kingston. A dark blue line additionally runs between Pawtucket and TF Green, making all local stops.

Is it pretty? No, not really. Is it even particularly clean or neat? No – my spacing of station names is inconsistent, the size of the station markers varies, and the bends of the lines are a bit sloppy.

But does it convey the information I needed to? Yes. Station are clearly and legibly marked (yes, I got tired of labeling the MBTA stations), the service patterns are clearly indicated (at least I hope they are clear – can you tell which stops the MBTA skips?), and there are enough nods to geographic fidelity (such as the curve through Providence) to provide context for the viewer. 

And while it’s true that a lot of the details are sloppy, the overall design is generally symmetrical and balanced, the stop spacing is mostly even, and the visual is streamlined and simplified – all of which means that even though the design itself is hardly inspired, it’s also not going to be too distractingly bad.

(To editorialize briefly: there is something to be said for transit diagrams and transit proposals which are straightforward enough to visualize using Paint.)

There are several downsides to using Paint, though. As mentioned above, you can get clear diagrams – “with some effort and care”. The time and effort is where you pay the price. It takes a while to do simple things nicely with Paint, and it takes a long time to do complicated things at all.

If you’re just getting started, I highly recommend playing around with Paint a little bit – get a feel for what “looks good” in the kinds of diagrams you’re making, but aim to “level up” once you can. 

Metro Map Maker

This website lets you make transit diagrams from scratch, or remix those created by other users. It is very simple to learn, and can be pretty fun to use. That said, I myself don’t use Metro Map Maker, and I think there are a couple of things to bear in mind if you choose to do so. 

First, I myself find the interface “fiddly” and difficult to draw things precisely the way I intend (which is my main reason for using a computer rather than sketching something by hand). I think this is visible in some of the maps in the gallery, some of which are quite clean and clear, and others of which struggle a bit. In particular, I have a hard time with the way Metro Map Maker renders Y junctions, where two branches feed into a single trunk; the way Metro Map Maker does it, it’s very easy to mistake those for three-way junctions.

The other thing to be aware of is that Metro Map Maker diagrams sometimes get poo-pooed by folks on aesthetic or “professionalism” grounds. There are some venues where such criticisms are appropriate – I would not, for example, recommend that transit advocates use this site to create visualizations of the services they are advocating for. But, especially if you’re just doing this for fun, I think it’s a perfectly reasonable tool. Just keep in mind who your audience is!

Paint.NET 

This is my application of choice these days, and the way I create most of the visualizations on this blog. (Unfortunately it is Windows-only, and I haven’t yet found a Mac equivalent that has the same “sweet spot” of features.) Paint.NET is available on its website and on the Windows store. 

For me, Paint.NET has an excellent balance of ease-of-use and useful-features. Eventually I plan to “graduate” into one of the programs listed below, but for now, I’ve gotten pretty darn good with Paint.NET, so I’m sticking with it for now.

One of the biggest reasons to use Paint.NET instead of MS Paint is the ability to use layers. For example, let’s say that I’m drawing up plans for Super High Speed Rail between Boston and New York, but I’m trying to decide between an alignment via Hartford and an alignment via New London. In MS Paint, I’d have to settle for a workaround: draw both using different colors, make a copy of my file after drawing the Boston-Providence and NYC-NH segments and work on two separate diagrams, or just pick one and forget about the other. In Paint.NET, on the other hand, I can just create a new layer for each alternative, and show/hide them depending on which version I want to look at.

You can also use layers to add and hide different levels of detail. For example, in one of my big maps, I have the local bus routes drawn in a layer of their own; I keep it hidden most of the time to reduce clutter, but sometimes show it in order to see things in full context.

I also recommend isolating lines, station markers, and labels each into their own layer. This will make it easier later on to make quick changes en masse, such as changing the color of a line.

Paint.NET still has downsides. It produces raster graphics and not vector graphics, which means things are pixellated when zoomed, and you actually physically cannot create pure curves (although if you are zoomed out reasonably, no one will notice the difference aside from professional designers). And as far as I know, Paint.NET has no concept of “linked objects”, meaning that if you create a fancy icon as a station marker, you’ll have to copy and paste it each time you want to use it, and you’ll have to manually modify every single copy if you want to make changes later. (This is why I just use a simple black circle for my station markers.)

Overall, however, I’ve found Paint.NET to be extremely useful.

Inkscape and Adobe Illustrator

Now we have “graduated” into the top tier: vector graphics. These are the programs used by professionals, and damn can you make things look good with them! Vector graphics allow you to infinitely zoom without pixelation and draw perfect curves without jagged edges. 

Adobe Illustrator is part of the Adobe Creative Cloud suite (along with the perhaps more famous Adobe Photoshop program). Inkscape is open-source, and available for free. 

Becoming more comfortable with Inkscape is a medium-term goal of mine, but it’s not a tool that I am experienced with at this point. So, for once, I don’t have much to say on the topic. But if you get the opportunity to practice with either of these programs, I highly recommend it! 

What about Google Maps?

Good question. You also may have questions about using Paint.NET specifically, or what I’m talking about in the footer of my blog about “basemaps” (and why I have that footer in the first place), or why I’ve been talking more about “diagrams” than about “maps”. All great questions! I hope to get to them in a subsequent post. In the meantime, hopefully I’ve given curious readers enough to get a running start! Have fun crayoning!

Extending the T’s Blue Line west: First Phases – Reaching the “Inner Belt”

In this and the following post, I’ll go through the Blue Line West extension proposals that are commonly discussed these days.

Route 128 is well-accepted as the limit of what might be called “Metro Boston”, and typically is the outer limit of most transit proposals, and has been for decades, even before Route 128 was built. It sits 10 miles outside of downtown, which often is the limit of a rapid transit system’s reach, in cities around the world.

There is a much fuzzier “Inner Belt” that encircles what we might call “Greater Downtown”, encompassing not only the Financial District, but also the built-up areas of the Seaport, Back Bay, Longwood, and Kendall. This is definitely not a hard-and-fast delineation, but there is a subtle but noticeable shift in character for both the city and the transit network as you cross through the “inner belt”.

The name “Inner Belt” comes from a thankfully-cancelled proposed interstate highway (the remnants of which can be seen in the Inner Belt District and Inner Belt Road in Somerville). Its approximate path was also used in various plans for the Urban Ring. It sits approximately 3 miles outside of downtown, which is also a very typical location for circumferential rapid transit routes. The Inner Belt is roughly demarcated by a series of major transfer hubs: JFK/UMass, Nubian, Ruggles, Kenmore, Kendall or Central, Lechmere, Sullivan, and Maverick. Most of these are the first major transfer hubs outside of downtown on each route. 

Any expansion of the Blue Line beyond Charles will come in phases, and the first phase will be about carving a path to the Inner Belt. The second phase will be about where to go after that. This post will go through the major “first phase” proposals in common circulation these days.

Blue to Kenmore

The Boylston Street Subway isn’t the only way to reach Kenmore. Once again, everything old is new again, and we find inspiration from an early 20th century proposal:

A 1910 BERy map showing the East Boston Tunnel, Washington Street Tunnel, Cambridge subway (terminating at Park), the Atlantic Ave El, and a proposed "Riverbank Subway", originating at Charlesgate & Beacon St, heading east along the Charles River, then tunneling under Chestnut St in Beacon Hill, before curving over the Cambridge Subway and terminating in a loop at the northern end of Boston Common, integrated into Park Street Station
https://commons.wikimedia.org/wiki/File:1910_map_of_Boston_subway_including_proposed_Riverbank_Subway.jpg

The proposed “Riverbank Subway” would have run from Park Street (upper level) under Chestnut Street to what is now Storrow Drive out to Kenmore. Note that at the time, the Boylston Street Subway did not exist — the Riverbank would’ve been the access point for trolleys from Watertown, Allston, and Brookline. 

The modern incarnation of the Riverbank Subway proposal sees the Blue Line extended to Charles/MGH, down along the Esplanade and picking up Storrow out to Kenmore (with a quick jog over to Beacon after Mass Ave). In my opinion, you could see anywhere from 1 to 3 stations between Kenmore and Charles/MGH — definitely one at Mass Ave, and then perhaps one or two serving the far ends of Beacon Hill and/or Back Bay.

A map of the current MBTA rapid transit system, focused on Back Bay, showing the Blue Line extended to Charles/MGH along the Charles River to Kenmore, with stops at Arlington St (near the Hatch Shell), Exeter St, and Mass Ave

This is my preferred extension for the Blue Line. Kenmore is a major transfer hub and will become all the more so as the Urban Ring concept continues to be quietly implemented. An extension to Kenmore would then also provide relief to the Green Line.

While I believe this route also has the fewest problems of the alternatives, it does still present challenges. A subway under Storrow wouldn’t need to contend with too many utilities, and since it’s landfill there should be fewer 17th-century surprises underneath; however, that same landfill can be tricky to work with and would complicate construction.

Additionally, and I’ll discuss this further in my next post, there isn’t a clear “next step” beyond Kenmore for further extension. It’s not quite a dead-end, but it’s less clear than some of these other proposals would be.

An expansion on the previous map, using dotted lines to show possible extensions south through Longwood on the Riverside Branch, and west to Allston/Brighton via Commonwealth or via the Mass Pike, or north to Harvard Square via Allston.

Blue to Cambridge

Blue-to-Kenmore definitely does not enjoy unanimous support among enthusiasts like myself. The most common alternative is to send the Blue Line over to Cambridge — either directly under the Longfellow, or sometimes at an angle toward Binney Street. Once in Cambridge, there are a number of options. The most common I’ve seen is to turn west after Kendall to serve MIT and Cambridgeport. From there, you might cross back over to Kenmore, or head directly into Allston and beyond. 

To be clear, I think these ideas have merit and should be seriously considered. Challenges to overcome include a subway under the Charles — and in fact in all likelihood, a second subway as well to cross back again –, the landfill around MIT (most of the Grand Junction would be hard to tunnel under, due to the landfill underneath), avoiding redundancy with the Red Line, and lack of a good clear “landing point” at the end (the equivalent of Kenmore, in the example above). Of these, the biggest to me is the need for two subway crossings under the Charles, which to be honest I really have no idea about the feasibility or cost of. 

I also worry a bit about redundancy with the Red Line; Cambridgeport aside, Cambridge generally isn’t underserved by rapid transit, and I do believe there are equity and justice aspects to consider. 

Blue-to-Cambridge alternatives can be distinguished by their subsequent destinations:

Kenmore

A map of the current MBTA rapid transit system, showing Back Bay, Longwood, Brookline, Allston, and Cambridge. The Blue Line is extended to Charles/MGH and then across the river to Kendall/MIT, where it then turns under the Grand Junction, stopping at Mass Ave, and then crossing under the Charles River again to land at Kenmore. Dotted lines mark possible extensions south to Longwood via the Riverside Branch, west to Allston/Brighton via Commonwealth or via the Mass Pike, or north to Harvard Square via Allston.

Crossing back over to Kenmore is tempting, given that Kenmore is and will become more of a “gravitational center” for transfers. But a pair of lengthy river crossings seems hard to justify. If Kenmore is your destination for the extension, I think the Riverbank option makes more sense. 

BU Bridge

A map of the current MBTA rapid transit system, showing Back Bay, Longwood, Brookline, Allston, and Cambridge. The Blue Line is extended to Charles/MGH and then across the river to Kendall/MIT, where it then turns under the Grand Junction, stopping at Mass Ave, continuing on with a stop at Cambridgeport near Amesbury St, and continuing across the BU Bridge. Dotted lines mark possible extensions west to Allston/Brighton via Commonwealth or via the Mass Pike, or north to Harvard Square via Allston.

Carrying on straight to the BU Bridge makes for a more direct route. However, missing Kenmore is a significant loss. Moreover, all future alignments from BU Bridge could be accessed via a Riverbank subway to Kenmore. What you get uniquely from this alignment is HRT service to Cambridgeport. Given that LRT service is much more achievable for Cambridgeport and would likely satisfy the need there effectively, this seems like a less efficient alternative. 

Central

A map of the current MBTA rapid transit system, showing Back Bay, Longwood, Brookline, Allston, and Cambridge. The Blue Line is extended to Charles/MGH and then across the river to Kendall/MIT, doubling the Red Line to Central. Dotted lines mark possible extensions west to Watertown via Western Ave & Arsenal St or Cambridge St, the Mass Pike & N Beacon St, to Newton Corner via Cambridge St & the Mass Pike, or to Allston/Brighton via Cambridge St.

Doubling up to Central Square and then heading west has, on paper, some real strengths. Unlike Kendall, Central is a legitimate feeder bus transfer hub of its own right. From Central, further extensions appear ripe (on paper) to Watertown, Allston, and/or Brighton, following existing higher-ridership bus routes, on near-straightaway alignments.

The strongest argument in favor of an extension to Central Square is the access to Watertown. Access to Watertown via the Mass Pike ROW is shared between the Central Square alignment, the BU Bridge alignment, and the Kenmore alignment, but access via Western Ave & Arsenal St (the route of the 70 bus) is unique to the Central Square alignment.

Doubling up all the way to Harvard yields a similar set of pros and cons as Central does, although Harvard makes it much more difficult to continue on to Allston and Brighton.

Doubling the Red Line

Doubling along the Red Line in general is tempting, as a way to increase capacity. But I think this is a situation where the details of the build have outsized importance. In New York City, 4-track subways are mostly constructed on a single level, with shared platforms between services going in the same direction; if you are heading uptown, you go to the uptown platform and have the benefit of choosing the express vs local service based on which train arrives first.

If you aren’t able to unify services on single platforms, you run into drawbacks very quickly that will severely undercut your hoped-for capacity boost. For example, I’ve seen proposals that place a Blue Line station at Kendall off-set to the north, near the Volpe building. This may be a useful footprint in which to build a station, but it means that the Blue and Red Line platforms will be some distance apart.

Distanced platforms means that riders will need to commit to a line decision very early in the boarding process. This means that agnostic riders — those destined for Charles/MGH, Park, Government Center, State, DTX — will still have to pick a line, even though either one will work for them. They won’t have that New York-style “stand on the platform and grab the first train to arrive” option. Given that Red will eventually have 3-min headways and Blue (as far as I know) will have 5-min, you also won’t see an even split among agnostic riders, as some will prefer the higher-frequency service – the Blue Line will probably get very few agnostic riders, and instead will be only picking up riders who were destined for the Blue Line anyway.

It’s true that doing so would still relieve crowding on the Red Line, but it still is targeting only a particular subset of riders. Additionally, depending on how far the Blue Line doubles (Kendall, Central, Harvard), you’ll still need to contend with riders transferring from the Red Line (e.g. from Davis) to the Blue Line – where will they make the jump from Red to Blue (and therefore where could we expect crowding relief to start)? Charles/MGH looks like it will be very well-designed for easy transfers, so if Charles provides a shorter walk than Kendall does, many riders will continue to stay on the crowded Red Line, even though in theory the Blue is available.

I should also note that doubling up the Red Line would entail significantly more complicated tunneling than other proposals would.

There are other ways to address capacity on the Red Line. Increasing headways will add about 50% additional capacity. Extending the Green Line to Porter will siphon off both downtown and Longwood/Back Bay commuters, looking for a faster and/or one-seat ride to their destination. Providing circumferential service – for example LRT on the Grand Junction, or a Green Line spur to Harvard – will also reduce the number of riders needing to go downtown. Doubling up with the Blue Line is not the only option.

A verdict?

As mentioned above, I believe a first phase extension to Kenmore is the stronger option. I will discuss my reasoning in further detail in the final post in this series. That being said, I firmly believe that both alternatives present benefits and drawbacks, and neither presents a perfect solution.

Both alignments require study. As I laid out in previous posts, we’re in uncharted waters here. No heavy rail service has ever existed along the trajectory we are discussing, making this corridor almost unique among transit expansion proposals in Boston. So I absolutely want to be clear that a firm verdict is both impossible and undesirable at this stage.

What I will state with greater confidence is what options are created and eliminated by each of these alternatives. And what I will go into further depth on is how these alternatives reflect larger systemic choices we need to make as we envision the T’s future in the third millennium. This isn’t just about job growth in Kendall, or passenger transfers at Kenmore: it’s much bigger than that.

In the next post, I’ll talk about where the Blue Line might go after a Phase 1 extension.

Extending the T’s Blue Line west: Ideas of Yestercentury

Introduction

Proposals to extend what would become the MBTA’s Blue Line west of downtown have been floating around for over a hundred years. While most of these ideas have fallen out of the present discourse, they remain worth discussion, as they continue to inform today’s proposals.

Blue Eats Boylston Subway

Last week, I explained a 1926 proposal that would have linked the East Boston Tunnel to the Boylston Street Subway, freeing up the Tremont Street Subway to serve a dedicated line running from Huntington to Lechmere. Let’s quickly run through the challenges with enacting the 1926 proposal today.

A 1926 map of the Boston Elevated Railway system, with extant routes drawn in black, and two new routes highlighted in color: a yellow line, stretching from Maverick to Scollay to Boylston to Kenmore to Commonwealth to Allston, and a pink line, running from Lechmere to Boylston to Back Bay station to Huntington to Brigham Circle
https://commons.wikimedia.org/wiki/File:Map_of_1926_proposal_for_Boston_rapid_transit_lines.png

First, the Green Line now has four branches that fan out across a wide area. It would be impractical to create so many branches of the Blue Line. Even if the E Line were rerouted into the old Pleasant Street Portal, it’s still not feasible to convert the B, C and D Lines all to HRT. The original plan in 1926 was for the Beacon, Commonwealth, and Watertown trolleys to terminate at transfer stations at Kenmore and in Allston, mirroring similar designs in place at Harvard, Forest Hills, Dudley (now Nubian), Sullivan, and others. But in the intervening century, residents in Brookline and Allston have become accustomed to a one-seat ride to downtown, so short-turns with a transfer are much less politically feasible than they once were.

Additionally, there are a large number of corridors throughout Greater Boston that are more appropriate for LRT than HRT. Maintaining the Boylston Street Subway as LRT would provide a core piece of infrastructure for that network, benefitting the system overall. Conversion to HRT would hamper that effort. 

Finally, the turn at Boylston is too sharp for HRT vehicles. So not only would Park Street need major reconstruction to hook in the Blue Line, but Boylston would either need to be rebuilt, or altogether bypassed. At that point, you are basically rebuilding everything between Arlington and Haymarket, as opposed to reusing the existing infrastructure.

Blue to Huntington

This idea and ones similar have floated around for a while, popping up here and there, so it is worth mentioning for completeness. This one runs into the same challenges as the original 1926 proposal, in that you have to dig up the Common and then figure out what to do south of Park Street.

However, a roundabout route via Charles/MGH, Arlington St, and something like Stuart St could potentially be more feasible (as diagrammed below). The construction would be more complicated, and the alignment as the subway turns west past Arlington would be tricky — HRT can’t make the sharp turns that LRT can. Additionally, the roundabout route takes about 1.3 miles to go the ~.75 miles that the crow flies between Arlington and Government Center, which is makes for a substantial diversion.

A map of today's MBTA rapid transit system, focused on Back Bay, Longwood, and Brookline. The Blue Line extends from Bowdoin to Charles/MGH, down Storrow Drive to the north end of Arlington Street, runs along the Public Garden to the Green Line's Arlington station, before vaguely turning west under Stuart St (the diagram is imprecise) until reaching Huntington Avenue, where it continues until the Mission Hill neighborhood. From there, dotted lines show possible further extensions south to Forest Hills, and north to Brookline and then Harvard or Brighton

For those reasons, I see this route as implausible. That being said, if it could be swung, it would be a monster success. The ridership on the E Line is ridiculous, and you’ll notice that even on the 1926 map, they were proposing HRT under Huntington. Longwood is a beast of an employment center, and that’s not even considering the schools and the Pru. 

That being said, I believe that an extended subway under Huntington Avenue, running modern LRT (like Los Angeles’s) can effectively meet the needs of this corridor, and can do so as part of a larger more robust network that would see more one-seat rides into this neighborhood. In that respect, HRT is both unnecessary and would constrain the rest of the system.

[EDIT] Looking again, I noticed that leveraging Columbus Ave between Arlington Street and the Mass Pike could enable you to tunnel from Arlington Station to Back Bay Station using cut-and-cover and keeping the turn radius moderate enough that HRT could probably swing it.

A map of today's MBTA rapid transit system, focused on Back Bay, Longwood, and Brookline. The Blue Line extends from Bowdoin to Charles/MGH, down Storrow Drive to the north end of Arlington Street, runs along the Public Garden to the Green Line's Arlington station, before turning southwest under Columbus Ave, and then turning west next to the Mass Pike, stopping at Back Bay and continuing until reaching Huntington Avenue, where it continues until the Mission Hill neighborhood. From there, dotted lines show possible further extensions south to Forest Hills, and north to Brookline and then Harvard or Brighton

This route would still be pretty roundabout between Back Bay and Government Center. And I still maintain that Huntington is a better fit for an LRT trunk subway. However, it could be more doable than I originally thought.

[END EDIT]

Blue Eats GLX

Once upon a time, when the T and the state were reviewing alternatives for providing service along what is now the Green Line Extension, one idea considered was extending the Blue Line instead. There is some logic here — the GLX corridor probably could merit HRT.

There are some major downsides though. Beyond the obvious — GLX is finally nearing completion and it seems silly to be already talking about ripping it out and replacing with HRT — turning the Blue Line north after Bowdoin would eliminate any possibility of a Red-Blue Connection, which is sorely needed and pretty much must be included in any Blue Line Extension. Realistically, Blue-to-Charles is going to be built before any of these further proposals will ever be seriously considered, so we should assume Charles/MGH as the launching point for all such proposals.

One alternative that could perhaps be feasible on a fifty-year timescale would be to extend the Blue Line to Kendall, and from there turn north to pick up either or both of the GLX branches. Depending on how the Green Line evolves in the next few decades, it may be advantageous to provide some relief to its northwest quadrant.

A map of the current MBTA rapid transit system, focused on eastern Cambridge and Somerville, showing the Blue Line extended from Bowdoin to Charles/MGH, across the Longfellow Bridge to Kendall/MIT, after which it turns north under the Grand Junction, with a stop at Cambridge Street, before meeting the alignment of the present-day GLX Medford Branch, and carrying on from there; a dotted line indicates an optional extension to Union Square and beyond

There are some things I actually like about this idea — there’s a certain symmetry to it that I find aesthetically pleasing, for lack of a better term. There are specific challenges — for example, I think it would be difficult to find a good spot for a Green-Blue transfer station — but unfortunately the biggest challenge is still that it’s way too early to be planning another massive construction project on the GLX corridor.

Fifty or sixty years from now, if GLX trains are packed to the brim, and the Blue Line still terminates at Charles/MGH or Kendall/MIT, then this idea might merit closer consideration.

[EDIT] In conversation with user Brattle Loop on ArchBoston, I realized that a better path from Kendall/MIT to the GLX ROW would be via Inman and Union Squares. This would require significantly more tunneling — cut-and-cover to Inman, and then bored tunnel through Union Square and north to the GLX ROW. However, it would allow for a strong Green/Blue interchange at Union Square, potentially also with circumferential LRT service from Union to Sullivan and beyond. It would also provide a one-seat ride from Union to Kendall — hardly a critical pairing, but still one that is not possible via the Grand Junction. (I have also since been reminded that the Grand Junction is probably less-than-ideal for actual tunneling, given its landfill history. If Blue-Eats-GLX is ever to happen, the Union Square alignment is probably the way to go, though it would be expensive.)

A map of the current MBTA rapid transit system, focused on eastern Cambridge and Somerville, showing the Blue Line extended from Bowdoin to Charles/MGH, across the Longfellow Bridge to Kendall/MIT, after which it turns north under Hampshire Street, to a station at Inman Square, and then to a station at Union Square, before meeting the alignment of the present-day GLX Medford Branch at Gilman Square, and carrying on from there; a dotted Gold Line indicates potential circumferential service to Sullivan from the Grand Junction line and from Porter Square; the Grand Junction branch of the Gold Line has stops at Twin City Plaza, Cambridge St, Binney St, Main St/Kendall, and Mass Ave/MIT

[END EDIT]

Blue Horseshoe

One of the major gaps in the MBTA’s rail network is Chelsea and Everett. Most proposals suggest adding rapid transit along the commuter rail line, running circumferentially between the Orange and Blue Lines. Typically this is suggested as LRT or BRT, and indeed Chelsea (in theory) now sits on the MBTA’s rapid transit network thanks to the efforts at BRT on the SL3 branch; there are also official discussions afoot to extend SL3 to Everett.

I sometimes see folks — especially when working on their first “crayon map” of possible extensions — who propose branching either the Blue Line or the Orange Line to serve this corridor:

A map of the current MBTA subway network, focused on Chelsea and Everett, showing dashed line branches of both the Blue and Orange Lines, originating at Maverick and Sullivan respectively, following the historic ROW of the Grand Junction across the rivers into Chelsea and Everett, paralleling the Eastern Route Commuter Rail line

Neither of these are a “Blue Line West” extension proposal, so I won’t go into detail here, but neither of these branches are likely to ever get built. Right now, the T runs as many Orange Line trains as it can to Wellington and Malden Center, and as many Blue Line trains to Wonderland (and hopefully someday Lynn) as it can. It does this because those are all massive bus transfer nodes with huge ridership. Branching either the Blue or the Orange means that the transfer nodes in question will get half as many trains, and that’s just not feasible.

Every so often, someone rightly points out that we could do a moonshot and build a radial line to Chelsea, particularly if we could somehow integrate it to the Tobin Bridge. Radial service to Chelsea has been occasionally discussed, for over 100 years. See for example page 76 and 77 in the 1910 report of the Boston Transit Commission and  pages 71 through 73 in the 1914 report of the Boston Transit Commission. (Boston in Transit has a scan of the map from the 1914 report.) Vanshnookenraggen also refers to a proposal for an elevated line to Revere via Chelsea and Charlestown on his historic track map, though I haven’t been able to locate the original source for this. (Van, if you see this and can share your source, I’ll update the post!)

One wild idea for a radial line would be to build it from Charles/MGH via Science Park and Charlestown, alongside or possibly on the Tobin Bridge. To my knowledge, the first person to propose something like this was user davem on ArchBoston, though his proposal lacked the horseshoe element, and instead turned Charles/MGH into a traditional diverging point. I think the diverging alignment isn’t a feasible service pattern, so I’ve drawn a horseshoe instead that takes the idea but creates an unbroken line of track from Chelsea to Charles to Maverick. 

Once Chelsea is reached, one could continue on all the way to Lynn, though that would open its own share of complications. One could instead press north into Chelsea, and/or veer over to Everett. The challenge with both of these is that neither has seen rail, so there are no existing ROWs to leverage; the closest equivalent is the Route 1 highway. Rapid transit can built along highways, but it’s not ideal; transit should go close to where people live, and people try to avoid living near highways. A Blue Line extension would likely need to be a subway, with all the complication and cost that entails. 

Plus, there’s still the challenge of getting to Chelsea. The Tobin Bridge aside, how do you get to the Tobin? On paper, it is true that Charles St between Charles/MGH and Science Park is pretty much straight-shot aligned with Route 1 on the other side of the Orange Line, but the challenges become apparent in three dimensions — between Science Park and Route 1 is a spaghetti of highways, ramps, Orange Line subway tracks, commuter rail tracks, a dam, and the Charles River. 

On top of that, as mentioned above, you need to have a Red-Blue connection, and that basically has to be at Charles. Hence the horseshoe: Government Center-Bowdoin-Charles/MGH-Science Park. But even this alignment still leaves you lacking a connection between the Chelsea branch and the Orange Line.

My feeling is that there are more cost-effective solutions to serving Chelsea and the North Shore, and stronger cases for the Blue Line elsewhere.

In the next post, we’ll start to examine proposals that are more frequently discussed in contemporary planning and crayoning. 

Extending the T’s Blue Line west: Historical Background

Introduction

The MBTA’s Blue Line is one that has frequently captured the imagination of amateur transit planners. A proper HRT line like the Red and Orange Lines, but much shorter, and terminating in downtown Boston. Proposals to extend the Blue Line to the northeast to Lynn and beyond have been floating around for most of a century now, and the so-called “BLX” project is one that is alive and well and commonly discussed in the domain of “serious” proposals.

At the other end, there is an active proposal to extend the Blue Line west the short distance to Charles/MGH, to provide a badly needed connection to the Red Line. (This is the rare extension that is entirely missing from BERy’s proposed expansion map from 1945.) Personally, I think this is pretty likely to be built in the next 15 years, and I suspect it will be the next major project that advocates focus on once GLX finally opens.

For amateur transit planners, that terminus at Charles/MGH beckons for extension. “It’s a whole untapped HRT line!”, we say to ourselves. “With its own tunnel through downtown and to the airport to boot! Surely it’s worth extending it… somewhere.”

This post is the first in a series that will examine the different possibilities for extending the Blue Line west from Charles/MGH. Today’s post reviews some historical background that is relevant for this topic. Subsequent posts will review specific proposals, and the benefits and challenges of each. 

Why isn’t there an obvious answer?

The first thing to acknowledge is that there is not an obvious answer for where the Blue Line should go ​​west. Though it may not seem like it, this is actually a big deal. For all other major endpoints, there is general consensus for feasible corridors:

  • Wonderland: northeast to Lynn and beyond
  • Oak Grove: north to Reading 
  • Medford/Tufts: north toward Winchester
  • Union Square: northwest toward Porter and beyond
  • Alewife: northwest toward Arlington and/or west to Waltham
  • Riverside: no further extension, as you’ve already reached Route 128
  • Forest Hills: southwest to Roslindale Village and/or south to Readville
  • Ashmont: convert the Mattapan High Speed Line to HRT
  • Braintree: no further extension (already reached the proverbial Route 128 boundary, even though, yes, it’s technically not Route 128 at that point)

That brings us back to that 1945 map. By and large, these corridors have been identified for a century, and are closely tied to the paths Massachusetts’ original railroads struck out in the 19th century. 

But as you can see, there is no vision for where the Blue Line should go in 1945. 

1945 map by the Boston Elevated Railway company, showing proposed extensions of the rapid transit network, to Lynn, Reading, Woburn, Arlington, Riverside, Needham, Dedham, Milton, and Braintree
https://commons.wikimedia.org/wiki/File:1945_BERy_extensions_map.jpg

Now, to be clear, I am not saying that there’s nowhere for the Blue Line to go simply because transit planners in 1945 had no ideas. But it gives us a clue as to why there isn’t an obvious corridor — and for that, we need to look further back into history.

The Blue Line is half a line — and the Green Line has the other half

A brief refresher: the Tremont Street Subway opened in 1897 between Boylston and Park, enabling the street above to be liberated from the crush of streetcars coming from all over the city. The rest of the subway opened, running from Haymarket to Pleasant Street (south of today’s Boylston station), providing a north-south spine for streetcar service through downtown. In 1906, a tunnel opened between downtown and East Boston — today used by the Blue Line, but originally used by streetcars, providing a east-west complement to the Tremont Street Subway.

In the early 1920s, the East Boston Tunnel was converted to “rapid transit”, as they called it at the time: platforms were raised to high level, streetcar routes were truncated to a transfer station at Maverick, and heavy rail rolling stock — like those used on the Cambridge-Dorchester Subway and the El — replaced the streetcars in the tunnel. BERy was seeking to replicate successful models used at Harvard, Sullivan, Dudley (now Nubian), Forest Hills and others: maintain streetcars as local feeder services into large transfer stations where passengers can be whisked downtown by rapid transit. 

Ideas to unify the Central Subway with a link to East Boston have been around since the subway was first envisioned. You can see on Vanshnookenraggen’s Flickr an early plan to hook the East Boston tunnel (in planning at the time) directly in to the subway, via the loop at Scollay Square. (I can’t remember right now exactly when this particular map dates from, but I believe it’s late 1890s.)

A paper map of the "Plan showing one of the proposed Routes for East Boston Tunnel", where the route extends out of the loop at Scollay Square, continues under Hanover street before cutting east under the Harbor, and arriving in East Boston under Lewis Street
https://www.flickr.com/photos/vanshnookenraggen/346212879/in/album-72157594460286528/

In the early ‘20s, at the same time that the East Boston Tunnel was being converted to Rapid Transit, efforts were underfoot to do the same on what would become the Green Line. Those efforts are worth a post of their own, but suffice it to say that plans were grand.

One such plan — dating from 1926 — is illustrative for our present question. In addition to converting both the proto-Blue Line and proto-Green Line to full HRT rapid transit, this proposal also called for hooking the East Boston Tunnel into the Central Subway by way of Park Street, and then take over the subway to Kenmore and beyond. (You can view the original high-res map on Wikimedia Commons.)

https://commons.wikimedia.org/wiki/File:Map_of_1926_proposal_for_Boston_rapid_transit_lines.png

As you can see, this proposal breaks the Green Line in half, and gives half of it to the Blue Line. And let’s be clear: then and now, the corridor from Longwood to Huntington to Back Bay to downtown to Lechmere absolutely merits a rapid transit line of its own. The Green Line of today pulls double duty, covering the role of at least two rapid transit lines, if not more. 

Broadly heading west from downtown, there are three broad corridors, each meriting rapid transit: Boylston Street, Huntington Ave/Southwest Corridor, and Washington Street. The 1926 proposal lines up the proto-versions of the Blue, Green, and Orange Lines, and launches them out to those three corridors, one by one. 

This proposal, had it come to fruition, would have created a very balanced network, with relatively evenly spaced “spokes” radiating out from downtown, and each major corridor covered.

So, as seen here, it is possible to design a Boston subway system with 4 “full” lines, each with fully developed “legs” running into downtown. Today’s network — 2 full lines (Red and Orange), 1 half line (Blue), and 1 half-ish line (Green, at least until GLX opens) — is not an inevitability. 

Since the 1926 plans never came to fruition, we instead had the Blue Line and Green Line each running into downtown and basically terminating there. The 1926 plans illustrate how reasonable it would have been to consolidate the existing Blue Line and existing Green Line into a single line, and thus illustrate how the Green Line has essentially been acting like the second half of the Blue Line.

(Imagine if the Red Line were two lines: A Crimson Line running from Cambridge to South Station, and a Purple Line running from Post Office Square to South Station to Andrew and beyond. Both lines terminate downtown in this example, but very logically could be stitched together, forming a natural corridor through the core. I’m arguing that the same is true of the Green and Blue Lines: just as the hypothetical Crimson and Purple Lines combine to make the Red Line, the Green and Blue functionally combine to create a single corridor through the core.)

The Green Line has taken over what would have been the logical western half of a full-build crosstown Blue Line. This is why there is no obvious “next step” beyond Charles/MGH: the Green Line has been taking care of the East Boston Tunnel’s western counterpart all this time.

Why can’t 2026 be like 1926?

By which I mean, why can’t we take the 1926 proposal and use it today? There are quite a few reasons, and many of them overlap with other challenges for crayoning the Blue Line west, which I’ll describe in subsequent posts.

The first and most immediate reason is that the 1926 proposal called for a tunnel under the Boston Common to connect then-Scollay Square Station with Park Street Station so that the “Blue Line” could hook into the Tremont Street Subway and Boylston Street Subway. While I wouldn’t want to say that a new tunnel under the Boston Common will never happen, it’s extremely unlikely. Moreover, hooking a new subway into Park Street would almost certainly require massive modifications to the existing station.

The larger reason, which is illustrative of the general challenges of planning a westward extension of the modern Blue Line, is that the Green Line is no longer the vaguely mirror image of the East Boston Tunnel it once was. The Green Line has become an entire subnetwork of its own, with potential for radical expansion, and a century’s worth of inertia behind the current design. It has grown beyond what it was in 1926, and can’t be quite so easily replaced.

In the next posts, we’ll go through different proposals from past and present, examining the benefits and challenges for each.