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:

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. 

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.