Transit lines branch. Core routes have more demand than outlying ones, so naturally trains and buses run on trunk lines in the core and then branch farther out, to match frequency to demand. I gave an overview of this years ago. This is both normal across nearly all significant transit systems, and good practice. In this post, I’d like to focus on the opposite kind of branching, which I am going to call reverse branching, when one outlying line splits into two core routes. This is much less common, but exists in multiple cities, and leads to problems including restrictions on capacity and disappointing ridership. Cities should avoid building new lines that reverse branch, and in one famous existing case, London’s Northern line, the city is working on changing the situation by building a new outlying branch.
London’s Northern line, as can be seen on the Underground map, has three branches to the north and two in the center, but just one to the south. The highest ridership demand is in the center, but because both branches feed into just one southern branch, there is less than full capacity on the central branches, about 20 trains per hour each, compared with 30 tph on the southern branch and 33 tph on the Victoria and Jubilee lines. As a result, Transport for London has made recurrent plans to split the line for good: one central branch (through the City of London) using the existing southern branch and two of the northern ones, and one (through Charing Cross) using one northern branch and terminating at Kennington, the junction with the southern branch. An under-construction extension of the line from Kennington to Battersea can then be tied to the Charing Cross branch. There is some NIMBY opposition from a member of Parliament representing a constituency on one of the northern branches, who would like her constituents to have one-seat rides to both branches, but most likely, Transport for London’s need for capacity will make the split inevitable once the Battersea extension opens, ending the reverse branching practice.
In New York, routes branch and recombine, and thus it is common to have trains of different colors (which only denote Manhattan trunks) running together on a branch in Brooklyn, Queens, or the Bronx. The single busiest entry point into the Manhattan core is via 53rd Street Tunnel (connecting to Queens Boulevard), technically a branch since it runs trains connecting to both the Eighth and Sixth Avenue Lines. This, again, causes capacity problems. It’s not so bad on the numbered lines, where four trunk tracks (the Manhattan express trunks, carrying the 2/3 and 4/5) recombine in a different way to four tracks in Brooklyn (pairing the 2/5), but the lettered lines’ reverse branching in Uptown Manhattan and Queens initially forced eight trunk tracks (the Sixth and Eighth Avenue services, the B/D/F and A/C/E) to converge to six branch tracks (the two Queens Boulevard express tracks via 53rd, and the four Central Park West tracks). New subway connections have replaced this situation with twelve trunk tracks (including the Broadway Line’s N/Q/R) splitting to ten, spreading the problem around but not dealing with the fundamental restriction on capacity. The under-construction Second Avenue Subway will connect to the Broadway Line and run Q trains, raising the number of lettered tracks Uptown and in Queens to twelve, but this will not be enough to disentangle the tracks and provide full capacity on each core track; see below for proposed examples.
In Delhi, the Green Line splits into short branches, to provide transfers to two different Metro trunk lines. As seen on the system map, the Green Line does not enter central Delhi, and the current setup allows passengers to travel to central Delhi via two different routes. However, the Phase 4 extension plan extends the one branch to go out of the city in a V-shaped direction (the light green Kirti Nagar-Dwarka Section 28 line on this map), and has an extension that may connect to the other branch (Inderlok-Indraprastha, colored ocher on the map) to connect it to central Delhi, which may cause a serious mismatch in demand on the outlying common segment.
Finally, in Tokyo, subway lines reverse branch in two locations. The Namboku and Mita Lines share their southernmost three stations and the tracks in between. Although most Tokyo subway lines, including Namboku and Mita, run through to commuter lines, which provide the normal kind of branching, the Mita and Namboku Lines only do so either to the north or via the shared segment, as seen on this map, constraining capacity. They run only 12 peak tph each, and have low ridership by Tokyo subway standards. The Fukutoshin and Yurakucho Lines are in a similar situation, but the Fukutoshin Line does run through to a commuter line, the Tobu Tojo Line, without going through the shared segment (it is not depicted on the map, which is a few years out of date). The Fukutoshin Line has low ridership (see last page here), but the Yurakucho Line does not.
In all examples I’ve listed so far, the two core branches serve very central areas (as in London, New York, and Tokyo), or neither of them does (as in Delhi). Tokyo is somewhat of an exception, since the Yurakucho and Mita Lines serve Central Tokyo and the Fukutoshin and Namboku Lines serve secondary centers, but those secondary centers are very dense themselves; the Mita and Namboku Lines in particular are quite close in ridership. I am more wary of proposals to split an outlying line in the core that have one branch serving the CBD and one branch avoiding it, as in Delhi, assuming I understand the proposal correctly.
Also of note, all the examples I’ve listed involve subways. This is because conventional branching, with a core trunk splitting into multiple outlying branches, is more limited on urban rail than on both buses and regional rail. Most subway lines do not have more than two branches feeding into a trunk. In New York, not counting the split in the A, which is inherited from the LIRR, there is exactly one place where three subway routes share tracks: the N, Q, and R from Manhattan to Queens. In Stockholm, with its highly branched subway network, only one line, in one direction, splits into three. This is because even a split into three branches requires limiting off-peak frequency on the branches to less than a train every ten minutes, which is undesirable in large subway systems. The result is that reverse branching can easily create a situation in which there are more tracks in the core than in the outlying areas, as it does in all four cities surveyed above, restricting capacity on each core track.
In contrast, regional rail tends to operate at lower frequency on the branches, and this permits conventional branching with more than two branches per trunk. In addition, there are often turnback facilities at through-stations, and substantial four-track segments on otherwise two-track lines. The result is that reverse branching is possible without any constraint on core track capacity. The Berlin S-Bahn is highly branched in both the conventional and reverse senses. The RER E is being extended to the west, including a takeover of an RER A branch. And the Tokyo commuter rail network has extensive reverse branching, coming from through-service between commuter lines and subway lines but also from the Shonan-Shinjuku Line’s split from the Tokaido and Tohoku commuter lines. In none of these cases is there a significant restriction on core capacity, simply because there’s enough slack in the branches that they can’t fill to track capacity unless the core has filled as well.
In the US, I am familiar with three proposals for new subway lines that involve reverse splits, in Boston, Washington, and New York.
In Boston, the proposal actually involves commuter rail rather than the subway: the Worcester Line would use the Grand Junction Railroad to go through Cambridge to reach North Station, bypassing South Station. See map on page 38 of the statewide transportation capital budget proposal. This would not reduce capacity, since the Worcester Line is nowhere near exhausting the capacity of a two-track railroad, and moreover, the Grand Junction line would terminate at West Station within Boston proper, where there’s a railyard. However, this is still bad transit, for other reasons. West Station serves a residential neighborhood, without enough density to justify a fork toward both North Station and South Station. On top of that, since North Station lies outside the Boston CBD, the proposal is essentially a mixture of a radial and a circumferential line, with all the problems that would bring – and despite running as a circumferential line through Cambridge, there is no transfer planned with the Red Line, although the Grand Junction passes close to the Kendall/MIT station.
It would be better to bag all plans to use the Grand Junction until such time that the state builds the North-South Rail Link, connecting North Station with South Station. Then, the Grand Junction would make an almost perfect alignment for a circular line, with its eastern leg connecting North and South Stations and its western end going through Cambridge, making several stops, including a transfer to Kendall/MIT. This would require high investment – besides being a single-track at-grade line, the Grand Junction would require a new junction to connect to the Worcester Line to go east toward South Station, whereas today it only connects to the west, toward Allston and Brighton – but still a fraction of the cost of the North-South Rail Link, which is getting some serious political support, including from former governors Michael Dukakis and William Weld.
In Washington, there already is some reverse branching: the Yellow and Blue Lines share tracks in Virginia, but run on two different trunk lines in Washington proper, each shared with other lines, so four central tracks become four tracks in Virginia. But now with the opening of the Silver Line, raising the number of Virginia tracks to six, WMATA would like to separate the Blue Line from the Orange Line, which it shares tracks with in Washington, in order to provide six tracks across the District as well. This can only lead to awkward service patterns and wasted core capacity, as Matt Johnson demonstrates on Greater Greater Washington: because the Orange and Silver Line will keep interlining under any plan, reckoned from their split east there are only four tracks in Virginia and not six. Moreover, the Yellow Line interlines with the Green Line in the District, which means that even if it’s separated from the Blue Line, it could not run at full capacity.
Washington built itself into a corner with its Metro route decisions. There’s no corridor in the city that really needs a subway line; unlike New York, Los Angeles, and San Francisco, Washington has no corridor with so much bus ridership that it should be a subway line. A fourth subway line would be useful for service to Georgetown, but that’s about it. So decisions about a fourth line in the District should be based on the capacity needs of the branches, not those of the core. On a list of possible changes that WMATA looked at, Greater Greater Washington included a separated Silver Line, including separation up to the junction with the Orange Line so that they share no tracks. I’ll add that if WMATA wants to go down that route, then it should give the Orange Line its own route through the District and keep the Silver and Blue Lines together; this is because the Orange Line is the busiest of the three, so that it should be the least branched, in this case not branched at all whereas the other two do branch.
Finally, New York. Second Avenue Subway is going to change the nature of the reverse branching used by the lettered lines, for the better. Because the plan for Phases 1 and 2 is to run only the Q train, the city will finally have matching numbers of lettered tracks in and north and east of the Manhattan core: twelve tracks in the core, and twelve in Uptown Manhattan and Queens. Unfortunately, it is impossible to match service, because that would sever too many connections. Second Avenue Subway only connects to the Broadway express line, so to match service there couldn’t be any other service using the Broadway express.
Recall the London NIMBYism mentioned at the beginning of this post: that was about a service change that would give commuters a
cross-platform (see comment with diagram) transfer between their branch of the Northern Line and the central segment of the other branch. In New York, the transfers in western Queens involve a lot of walking between platforms, if they even exist. Then all the Broadway locals (the N/R) would go to Queens through 60th Street Tunnel, and thence to the Astoria Line, severing the connection to the Queens Boulevard Line. The Queens Boulevard Line has two ways into Manhattan: 53rd Street, which connects to both Eighth and Sixth Avenues, and 63rd Street, which connects only to Sixth Avenue. Moreover, either all F trains (through 63rd) have to run express in Queens and all E trains local, or the reverse; mixing and matching would produce at-grade conflict at the junction, as seen on the Queens 1 track map on nycsubway.org. The transfer between the E and F would be located at 74th Street in Queens, several kilometers east of the split, which is located just to the east of the westernmost express/local station, Queens Plaza. Neither the E nor the F would have a transfer to the N/R near their respective intersection points. The Q would not have a transfer to the E (it would have one to the F, though). This puts many commuters in an impossible situation and the capacity gains from it are frankly not enough to be worth it.
Instead, the capacity gains would be limited to running some more express trains on the Broadway Line. Before the service cuts in 2010, the N ran express on the Broadway Line, the Q terminated at 57th Street at the north end of Midtown, and a fourth Broadway route, the W, ran local and served the Astoria Line. Once the Q is extended up Second Avenue, the restored W could beef up Broadway Line service. Second Avenue would only get a branch despite its high ridership, but it’s still only a segment of a line.
Then there are Phases 3 and 4 of Second Avenue Subway, serving Midtown and the Lower East Side, where the Q runs on Broadway. The official plan introduces another reverse branching: a new route, the T, is planned to run the entire length of Second Avenue: see map here. When both phases are complete, there will be fourteen lettered tracks in Midtown but only twelve Uptown and in Queens.
To resolve this, the MTA should activate a connection that is included in the Second Avenue Subway plan as a non-revenue connection: a connection from Second Avenue south of the Q/T split at 63rd Street to 63rd Street Tunnel; currently, 63rd Street is the least used connection from Manhattan to Queens, since the reverse branching limits capacity and 63rd Street is the least useful connection since it enters Manhattan north of Midtown. This implies there should be a Queens Boulevard-Second Avenue service, which I will call the U, one letter next to the T. The Queens Boulevard express tracks are filled to capacity and the local ones are not, so the T should run local, cutting the frequencies on the existing local R and M trains a bit to make room. It would still leave New York with twelve Uptown and Queens tracks diverging to fourteen Midtown tracks, but it would distribute the load better, in the same way the present system distributes the load better than the 1930s-era reverse branching from six to eight tracks did.
New York is in a somewhat special case, in that its subway system is based on heavy branching and reverse branching, and moreover it’s historically based on three different systems, with poor transfers between them. Fully untangling the lines after Second Avenue Subway’s Phases 1 and 2 are built is not possible because there are no transfers between the lines that would result, and the station placement is such that any new transfers would involve long walks between platforms.
Other cities, especially cities planning new systems from scratch, should not emulate this feature, and should instead design all lines to either not branch at all or only branch conventionally. A system designed from the ground up could have cross-platform transfers between lines, and even make sure they’re timed, reducing the cost to passengers of having to transfer in lieu of using a reverse branch. It could be coherent, in the sense of making it easy for an unfamiliar passenger to understand how to get from each station to each other station. And it could be built for maximum capacity in the most crowded segments, where it matters the most.