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


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

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.


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


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


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

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

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.)


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.

Useful Things page created

I’ve created a page called “Useful Things“. There is an incredible wealth of information available online these days, some of it produced by official agencies, and some created by devoted enthusiasts. This list is my effort to draw attention to particular resources I’ve found useful.

Some of these materials are relatively well-known — Vanshnookenraggen‘s marvelous track maps, for example. Others are, I believe, more obscure — tucked away in massive collections, or legacies of earlier days of the internet. My list is hardly exhaustive or comprehensive, nor is it a “who’s who” of our transit enthusiast community; we are very fortunate to have such an active community, and I could never hope to list everyone.

If you have suggestions of other resources similar to the ones I’ve listed here, let me know!

MBTA Subway & Frequent Bus Network

Over the course of 2020, I undertook an exercise to identify all MBTA bus services which run at “rapid transit frequencies” at some point during the day. (I chose to focus on pre-pandemic schedules, under the theory that eventually we would return to those levels.) Unsurprisingly, I made a map, which can be viewed on Google Maps: MBTA Subway + Frequent Bus.

(More detail on my methodology and strategy here on ArchBoston.)

At a high level, what I found was that there were two layers of frequent bus networks: those that were frequent all day and those that were frequent at peak only. I termed the all-day services as “Gold Line” services, and the peak-only services as “Bronze Line”.

Turning on and off the Bronze layer on the Google Map reveals how the MBTA’s network “breathes” over the course of the day, in and out of peak periods. During the height of rush hour, a comprehensive hub-and-spoke network emerges, which then fades away in the mid-day, only to return in the afternoon.

Additionally, more notably within the Bronze network, multiple “cumulative corridors” appear when you stack several modest-frequency routes on top of each other. Washington Street south of Forest Hills, or Broadway in Everett, are good examples of this phenomenon.

Additional analysis included in my original ArchBoston post here. A deep-dive on the Dorchester network is available here, though hopefully I will give that topic a post of its own at some point.

To my knowledge, there is no other map or analysis that examines this particular topic. (If I am wrong, please let me know! I’d love to see that!) So I hope this is useful, or at least of interest, to someone out there.

Taken altogether, this map gives a truer picture of where Greater Boston’s high-frequency than either the Key Bus Route system (which aligns somewhat but not entirely with the Gold network) or the MBTA Bus Map (which is useful, but overwhelming). Take a look: play around with turning on and off the different layers, click into individual corridors to see their frequencies and constituent routes, and let me know about any corridors I may have missed!

The full network during peak hours.
The off-peak network, with routes that see high frequencies all day.
The rapid transit network.