A recent discussion on Twitter about the through-running plan offered by ReThinkNYC got me thinking about an aspect American through-running crayonistas neglect on their maps: the branch-to-trunk ratio. It’s so easy to draw many branches converging on one trunk: crayon depicts a map and not a schedule, so the effects on branch frequency and reliability are hard to see.
In contrast with crayonista practice, let us look at the branch-to-trunk ratio on existing through-running commuter networks around the developed world:
The RER has 5 lines, of which 4 are double-ended and 1 (the E) is single-ended, terminating in the Paris CBD awaiting an extension to the other side. They have the following numbers of branches:
RER A: 3 western branches, 2 eastern branches.
RER B: 2 northern branches, 2 southern branches; on both sides, one of the two branches gets 2/3 of off-peak traffic, with half the trains running local and half running express.
RER C: 3 western branches, 4 eastern branches; one of the eastern branches, which loops around as a circumferential to Versailles, is planned to be closed and downgraded to a tram-train.
RER D: 1 northern branch, 3 southern branches; the map depicts 4 southern branches, but only 3 run through, and the fourth terminates at either Juvisy or Gare de Lyon.
RER E: 2 eastern branches; the ongoing western extension does not branch, but is only planned to run 6 trains per hour at the peak, so some branching may happen in the future.
The RER B and D share tracks between Chatelet-Les Halles and Gare du Nord, but do not share station platforms.
Thameslink has 3 southern branches. To the north it doesn’t currently branch, but there is ongoing construction connecting it to more mainlines, and next year it will gain 2 new northern branches, for a total of 3. Crossrail will have 2 eastern branches and 2 western branches. Crossrail 2 is currently planned to have 3 northern branches and 4 southern branches.
Berlin has 2 radial trunk routes: the east-west Stadtbahn, and the North-South Tunnel. The Stadtbahn has three S-Bahn routes: S5, S7, S75. The North-South Tunnel also has three: S1, S2, S25. Each of these individual routes combines one branch on each side, except the S75, which short-turns and doesn’t go all the way to the west.
Berlin also has the Ringbahn. The Ringbahn’s situation is more delicate: S41 and S42 run the entire ring (one clockwise, one counterclockwise), but many routes run on subsegments of the ring, with extensive reverse-branching. At two points, three services in addition to the core S41-42 use the Ringbahn: S45, S46, and S47 on the south, and S8, S85, and S9 on the east.
There is a two-track central tunnel, combining seven distinct branches (S1-8, omitting S5). S1 and S2 further branch in two on the west.
The excessive ratio of branches to trunks has created a serious capacity problem in the central tunnel, leading to plans to build a second tunnel parallel to the existing one. This project has been delayed for over ten years, with mounting construction costs, but is finally planned to begin construction in 2 days, with expected completion date 2026. At more than €500 million per underground kilometer, the second tunnel is the most expensive rail project built outside the Anglosphere; were costs lower, it would have been built already.
The Tokyo rail network is highly branched, and many lines reverse-branch using the subway. However, most core JR East lines have little branching. The three local lines (Yamanote, Chuo-Sobu, Keihin-Tohoku) don’t branch at all. Of the rapid lines, Chuo has two branches, and Tokaido and Yokosuka don’t branch. Moreover, the Chuo branch point, Tachikawa, is 37 km from Tokyo.
The northern and eastern lines branch more, but the effective branch-to-trunk ratio is reduced via reverse-branching. To the east, the Sobu Line has 5 branches, but they only split at Chiba, 39 km east of Tokyo. The Keiyo Line has 3 branches: the Musashino outer ring, and two eastern branches that also host some Sobu Line trains. The services to the north running through to Tokaido via the Tokyo-Ueno Line have 3 branches – the Utsunomiya, Takasaki, and Joban Lines – but some trains terminate at Ueno because there’s no room on the Tokyo-Ueno trunk for them. The services using the Yamanote Freight Line (Saikyo and Shonan-Shinjuku) have 2 southern branches (Yokosuka and Tokaido) and 3 northern ones (Utsunomiya, Takasaki, and a third Saikyo-only branch).
Conversely, all of these lines mix local and express trains on two tracks, with timed overtakes, except for the three non-branching local lines. The upper limit, beyond which JR East only runs local trains, appears to be 19 or 20 trains per hour, and near this limit local trains are consistently delayed 4 minutes at a time for overtakes.
Implications for Through-Running: Boston
In Boston, there are 7 or 8 useful southern branches: Worcester, Providence, Stoughton, Fairmount, the three Old Colony Lines, and Franklin if it’s separate from Fairmount. The Stoughton Line is planned to be extended to New Bedford and Fall River, making 8 or 9 branches, but the intercity character of the extension and the low commute volumes make it possible to treat this as one branch for scheduling purposes. To the north, there are 5 branches today (Fitchburg, Lowell, Haverhill, Newburyport, Rockport), but there are 2 decent candidates for service restoration (Peabody and Woburn).
The North-South Rail Link proposal has four-tracks, so the effective branch-to-trunk ratio is 3.5. It is not hard to run service every 15 minutes peak and every 30 off-peak with this amount of branching, and there’s even room for additional short-turn service on urban lines like Fairmount or inner Worcester and Fitchburg. But this comes from the fact that ultimately, Boston regional rail modernization would create an RER C and not an RER A, using my typology as explained on City Metric and here.
There are several good corridors for an RER A-type service in Boston, but those have had subway extensions instead: the Red Line to Braintree, the Orange Line to Malden, and now the Green Line Extension to Tufts. The remaining corridors could live with double service on an RER C-type service, that is, service every 7.5 minutes at the peak and every 15 off-peak. For this reason, and only for this reason, as many as 4 branches per trunk are acceptable in Boston.
Implications for Through-Running: New York
Let us go back to the original purpose of this discussion: New York through-running crayon. I have previously criticized plans that use the name Crossrail because it sounds modern but only provide a Thameslink or RER C. Independently of other factors, the ReThinkNYC plan has the same issues. It attempts to craft a sleek, modern regional rail system exclusively out of the existing Penn Station access tunnels plus a future tunnel across the Hudson.
Where Boston has about 7 commuter rail branches on each side, New York has 9 on Long Island (10 counting the Central Branch), 6 in Metro-North territory east of the Hudson, and 9 in New Jersey (11 counting the Northern Branch and West Shore Railroad). Moreover, one branch, the Hudson Line, has a reverse branch; where the Keiyo/Sobu reverse-branching in Tokyo and the Grand Central/Penn Station Access reverse-branching on the New Haven Line offer an opportunity to provide more service to a highly-branched line, the Hudson Line is a single line without branches.
The upshot is that even a four-track trunk, like the one proposed by both the RPA’s Crossrail NY/NJ plan and ReThinkNYC, cannot possibly take over all commuter lines. The frequency on each branch would be laughable. This is especially bad on the LIRR, where the branch point is relatively early (at Jamaica). The schedule would be an awkward mix of trains bound for the through-running system, East Side Access, and perhaps Downtown Brooklyn, if the LIRR doesn’t go through with its plan to cut off the Atlantic Branch from through-service and send all LIRR trains to Midtown Manhattan. Schedules would be too dependent between trains to each destination, and reliability would be low. ReThinkNYC makes this problem even worse by trying to shoehorn all of Metro-North, even the Harlem and Hudson Lines, into the same system, with short tunneled connections to the Northeast Corridor.
On the New Jersey side, the situation is easier. This is because two of the key branch points – Rahway and Summit – are pretty far out, respectively 33 and 37 km from Penn Station. The population density on branches farther out is lower, which means a train every 20 or 30 minutes off-peak is not the end of the world.
The big problem is the attempt to link the Erie lines into the same system. This makes too many branches, not to mention that the Secaucus loop between the Erie lines and the Northeast Corridor is circuitous. The original impetus behind my crayon connecting the South Side LIRR at Flatbush with the Erie lines via Lower Manhattan is that the Erie lines point naturally toward Lower Manhattan, and not toward Midtown. But this is also an attempt to keep the branch-to-trunk ratio reasonable.
The first time I drew New York regional rail crayon, I aimed at a coherent-looking system. The Hudson Line reverse-branched, and I was still thinking in terms of peak trains-per-hour count rather than in terms of a consistent frequency, but the inner lines looked like a coherent RER-style network. But the Hoboken-Flatbush tunnel still had 5 branches on the west, and the Morris and Essex-LIRR line, without a dedicated tunnel, had 4 to the east. My more recent crayon drops the West Shore Line, since it has the most freight traffic, leaving 4 branches, of which 1 (Bergen County) can easily be demoted to a shuttle off-peak, keeping base frequency on all branches acceptable without overserving the trunk; by my most recent crayon, there are still 4 branches, but there’s a note suggesting a way to cut this to 3 branches by building a new trunk. Moreover, several branches are reduced to shuttles (Oyster Bay, Waterbury) or circumferential tram-trains (West Hempstead) to avoid overloading the trunks. There’s a method behind the madness: in normal circumstances, there should not be more than 3 branches per double-track trunk.
I am not demanding that the RPA or ReThinkNYC put forth maps with multiple new trunk lines. The current political discussion is about Gateway, which is just 1 trunk line; it’s possible to also include what I call line 3 (i.e. the Empire Connection), which just requires a short realignment of an access track to Penn Station, but the lines to Lower Manhattan still look fanciful. New York has high construction costs, and the main purpose of my maps is to show what is possible at normal construction costs. But it would be useful for the studios to understand issues of frequency, reliability, and network coherence. This means no Secaucus loop, no attempt to build one trunk line covering all or almost all commuter lines, and not too many branches per trunk.
New York is an enormous city. It has 14 subway trunk lines, and many are full all day and overcrowded at rush hour. That, alone, suggests it should have multiple commuter rail trunk lines supplementing the subway at longer-range scale. It’s fine to build one trunk line at a time, as London is doing – these aren’t small projects, and there isn’t always the money for an entire network. But it’s important to resist the temptation to make the one line look more revolutionary than it is.