Governor Kathy Hochul announced a policy package for New York, and, in between freeway widening projects, there is an item about the Triboro subway line, renamed Interboro and shortened to exclude the Bronx. The item is brief and leaves some important questions unanswered, and this is good – technical analysis should not be encumbered by prior political commitments made ex cathedra. Good transit advocates should support the program as it currently stands and push for swift design work to nail down the details of the project and ensure the decisions are sound.
What is Triboro?
The Bronx Times has a good overview, with maps. The original idea, from the RPA’s Third Regional Plan in the 1990s, was to use various disused or barely-used freight lines, such as the Bay Ridge Branch, to cobble together an orbital subway from Bay Ridge via East New York and Jackson Heights to Yankee Stadium. Only about a kilometer of greenfield tunnel would be needed, at the northern end.
In the Fourth Plan from the 2010s, this changed. The Fourth Plan Triboro was like PennDesign’s Crossboro idea, differing from the Third Plan Triboro in three ways: first, the stop spacing would be wider; second, the technology used would be commuter rail for mainline compatibility and not subway; and third, the Bronx routing would not follow disused tunnels (by then sealed) to Yankee Stadium but go along the Northeast Corridor to Coop City. Years ago, I’ve said that the Fourth Plan Triboro is worse than the Third Plan.
Unlike the RPA Triboro plans, Hochul’s Interboro plan only connects Brooklyn and Queens, running from Jackson Heights to the south. I do not know why, but believe this has to do with right-of-way constraints further north. The Queens-Bronx connection is on Hell Gate Bridge, which has three tracks and room for a fourth (which historically existed), of which Amtrak uses two and CSX uses one; having the service run to the Bronx is valuable but requires figuring out what to do about CSX and track-sharing. The Third Plan version ignored this, which is harder now, in part because freight traffic has increased from effectively zero in the 1990s to light today.
The stop spacing in the governor’s plan appears to be more express, as in the Fourth Regional Plan, where the service is to run mostly nonstop between subway connections. In contrast, the Third Regional Plan called for regular stop spacing of 800 meters, in line with subway guidelines for new lines, including Second Avenue Subway.
I’m of two minds on this. We can look at formulas derived here in previous years for optimal stop spacing; the formulas are most commonly applied to buses (see here and follow first paragraph links), which can change their stop pattern more readily, but can equally be used for a subway.
The line’s circumferential characteristic gives it two special features, which argue in opposite directions on the issue of stop spacing. On the one hand, trips are likely to be short, because many people are going to use the line as a way of connecting between two subway spokes and those are for the most part placed relatively close to one another; farther-away connections such as end-to-end can be done on a radial line. But on the other hand, trips are not isotropic, because most riders are going to connect to a line, and the stronger the distinguished nodes are on a line, the longer the optimal interstation is.
On this, further research is required and multiple options should be studied. My suspicion is that on balance the longer stop spacing will prove correct, but it’s plausible that the shorter one is better. A hybrid may well be good too, especially in conjunction with a bus redesign ensuring the stops on the new rail link are aligned with bus trunks.
The issue of frequency
The line’s short-hop characteristic has an unambiguous implication about service: it must be very frequent. The average trip length along the line is likely to be short enough, on the order of 15 minutes, that even 10-minute waits are a drag on ridership. Nor is it possible to set up some system of timed transfers to 10-minute subway lines, first of all because the subway does not run on a clockface timetable, and second because the only transfers that could be cross-platform are to the L train.
This means that all-day frequency must be very high, on the order of a train every 5 minutes. This complicates any track-sharing arrangement, because the upper limit of frequency on shared track with trains that run any other pattern is a bit worse than this. The North London Line runs every 10 minutes and shares track with freight, and I believe there are some short shared segments in Switzerland up to a train every 7.5 minutes.
The upshot is that freight can’t run during normal operations. This is mostly fine, there are only 2-4 freight trains a day on the Bay Ridge Branch, where there are segments of the right-of-way that are only wide enough for two tracks, not four. This means if freight is to be retained, it has to run during light periods, such as 5 in the morning or 11 at night, when it’s more acceptable to run passenger trains every 10 minutes and not 5.