Nearly all rapid transit lines belong to one of two categories: radial lines (a large majority), which connect city center with outlying neighborhoods or suburbs; and circumferential lines (a minority), which go around city center and often serve secondary centers and usually intersect all or nearly all radial lines perpendicularly, such as Paris’s Lines 2 and 6, Moscow’s Circle Line, Seoul’s Line 2, New York’s G train, and Shanghai’s Line 4. In this post, I’m going to discuss an uncommon third category, that of lines that combine circumferential and radial functions: they go toward city center, like a radial line, but then change direction and become circumferential. The G train in New York was like this until 2001, and Line 3 in Shanghai is like this today. This is apropos a proposal by a team Penn Design graduate students to build a variant of Triboro RX in New York that combines Triboro’s circumferential orientation with a radial commuter line. I believe such mixed lines are a recipe for low ridership and strained transfer points, and the Penn Design proposal is inferior to the original Triboro proposal.
First, some details about the mixed lines in question. The example most accessible to most readers is the historical G train in New York. When it first opened in the 1930s as part of the IND, it was designed to both connect Brooklyn and Queens without going through Manhattan and provide local service along radial lines, running alongside express trains that would serve Manhattan. Thus the northern half of the G ran under Queens Boulevard as a local, while the E and F trains provided express service and went to Manhattan. From the start, this arrangement was unstable. Demand for service to Manhattan was much greater than to Brooklyn, so people riding the G inbound changed to the E or F at the first express station after the one they boarded. With overcrowded express trains and undercrowded local ones, the Transit Authority was compelled to build a track connection in 1955 to add a Manhattan-bound local service, and to build a second track connection in 2001 to add another Manhattan-bound local train and remove the G from Queens Boulevard entirely.
In Shanghai, Line 3 was built as an (almost) entirely above-ground line, interlined for part of the way with the circular Line 4. The northern half of Line 3 is radial, running parallel to the overcrowded Line 1. However, where Line 1 enters the traditional center and serves People’s Square, Line 3 swerves west to go around it (missing Lujiazui, the new high-rise CBD to the east of People’s Square), interlining with Line 4, and leaving the loop southward to intersect Line 1 again at Shanghai South Railway Station. Its ridership disappoints not only by the standards of Line 1, but also by those of Line 4: 642,000 on 2014/4/30, the system’s busiest day, compared with 1,384,000 on Line 1 and 907,000 on Line 4. Line 6, which likewise combines a radial function at its northern end with a circumferential one at its center, serving Century Avenue but not Lujiazui or People’s Square, has even lower ridership, 376,000, although this is several times the original projection.
There’s a discussion on Human Transit, in which consensus is that the best circumferential lines connect secondary activity nodes that generate trips in their own right. Now, the G train connects Downtown Brooklyn (the largest business district in New York outside Manhattan) with Long Island City (one of the business districts of Queens), but it lacks the other positive feature of circumferential lines: transfers to the radial lines, to allow one-transfer trips from anywhere to those secondary nodes. The G has good transfers only to other IND lines, and at the Queens end, its transfer to the Queens Boulevard trains was cut in 2001 since, for operational reasons, it was cut not to its old junction with the E and F (Queens Plaza) but one station short (Court Square). Other G transfers are very recent and require a considerable amount of walking.
In contrast to the underperforming G, circumferential lines that both connect important activity nodes and have plenty of radial transfers are backbones of their cities’ transit systems. Shanghai’s Line 4 is fairly busy as noted above. Seoul’s Line 2 is according to a forum post the busiest in the system. Paris’s Lines 2 and 6 are only about average in ridership but combined would be the second busiest after Line 1 (and per route-km are third and fourth, only behind Lines 1 and 4, but are only narrowly ahead of many other lines). The juxtaposition of Shanghai’s Lines 3 and 4 in particular suggests that subway lines shouldn’t try to mix radial and circumferential functions.
Let us go back to the impetus for the post, Triboro RX. The proposal is to largely use existing freight rail lines, all of which are lightly used and could be turned over to the subway, to provide a semicircular line connecting nodes of activity in the Bronx, Queens, and Brooklyn. Because of the focus on using an existing right-of-way to reduce costs, the line misses the most important nodes in Brooklyn and Queens, which are served by the G in any case. However, it passes within half a kilometer of the Hub in the Bronx and, via a short greenfield tunnel, connects to Yankee Stadium, the Bronx’s busiest subway station; it also connects to Brooklyn College and 74th Street/Broadway in Queens, both busy stations if not as central as Downtown Brooklyn or Long Island City. Moreover, it provides direct Bronx-Queens service, which in the existing system requires circuitous routes through Manhattan with difficult transfers, and has reasonable transfers to nearly all subway lines. At the end, the lack of service to Downtown Brooklyn ensures it cannot be a very well-patronized line, but as the right-of-way is almost entirely in place, its cost per rider could be quite low.
In contrast, the Penn Design proposal, called Crossboro, severs the connection to the Hub and Yankee Stadium, and replaces it with service along the Northeast Corridor to Coop City, making sparse stops, at the same locations Metro-North plans to for Penn Station Access; this is 4 stops in 10 km in the Bronx, compared with stops spaced roughly every 800 meters in Queens and Brooklyn, as in the original proposal. The trains would be certified for mainline operation, on the model of the London Overground, rather than segregated from mainline traffic.
The problem with the Crossboro idea is essentially the same as that of the G train until 2001. Most riders at the four Bronx stops are interested in getting to Manhattan, and not to the neighborhoods served by Crossboro, so they’d look for transfer opportunities. The only such opportunity in the Bronx is at Hunts Point, to the 6, which unlike the E and F on Queens Boulevard runs local and provides slow service to Manhattan. With the extra transfer, there is no advantage to Bronx riders over continuing to walk or take the bus to the 2, 5, and 6 trains to Manhattan. Moreover, because of the poor transfers within the Bronx, it’s impossible to use the line to connect from Queens to anywhere in the western half of the Bronx, including Yankee Stadium and the Hub: there’s a connection only to the 6, and none to the 2, 4, 5, or B/D, or to Metro-North.
The principle in action here is that, especially when there are no compelling destinations, it’s critical to make sure the line provides connectivity between large regions. This means connecting to all or almost all radial lines in a region well-served by radial subways but poorly served by preexisting circumferential ones. Not including the 1, which no proposal connects to, the Bronx has five subway lines, all providing radial service to Manhattan; it is a feature of Triboro that it connects to all five (though the connection to the 2/5 requires a long walk), ensuring that people from nearly everywhere in the Bronx can use the line to get to its destinations in Queens and Brooklyn with one transfer. To get from anywhere to anywhere would require two transfers, but to get from a random station in the Bronx to one in Queens or Brooklyn often already requires two transfers, usually at busy Manhattan stations that are out of the way for the crosstown traveler.
Mixing radial and circumferential service interferes with this principle, since the radial component has to come at the expense of completing the circle (or semicircle in a geographically-constrained city like New York). Thus, it’s harder to use the line to get to a large enough variety of points of interest to make up for the fact that it misses the city’s most important destinations. Of course, such a line is also wanting as a radial line, since it misses the center. Thus, ridership underperforms, and the line usually fails to achieve its stated purpose.