The ideal rapid transit line looks something like a straight line. It can have deviations, but on a map it will be more or less a line with a definitive direction. Most rapid transit lines are indeed linear, or failing that circular (to provide circumferential service) or L-shaped. In most cities there are just a handful of C-shaped exceptions: London has just one (the Piccadilly Line), Tokyo two (the Marunouchi Line and the Yokosuka-Sobu Line), Paris one (the RER C; Metro 2 and 6 should really count as a circle), Seoul one (Line 6). In contrast, in some cities, such as New York, there are many C-shaped lines. Since most people aren’t traveling in semicircles, it’s worth talking about reasons why cities may build lines that don’t have the most efficient shape.
Reason 1: water
Cities right next to a large body of water may have lines that double back. Chicago has the Blue Line, Toronto has the Yonge-University-Spadina Line, San Francisco has the Daly City-Dublin and Daly City-Fremont BART routes and the T-Third Muni route. If Boston extends the Green Line to Somerville, the Green Line will form a C. Tokyo’s Yokosuka-Sobu through-line is in this category as well. Usually, the transit operator doesn’t expect anyone to take the line for its full length; Toronto is planning a crosstown line bridging the far ends of the C. Such lines are C-shaped because they are really two interlined lines coming from the same direction.
Reason 2: two separate lines joined at the outer end for operational reasons
This can be similar to reason 1 in that nobody is expected to take the line along its full length, but here the joining occurs at the outer end. Singapore’s North-South Line and Vancouver’s Millennium Lines are both examples of this. In Singapore’s case this comes from an international boundary; in Vancouver’s it comes from the need to connect the line to the Expo Line so that trains can go to the maintenance yard, and it proved too hard to connect the lines at the inner end, at Broadway/Commercial. In both examples, what should really be two separate lines are joined by an outer loop that functions as somewhat of a circumferential, but the lines were not planned to provide circumferential service and are not good at connecting to anything other than the two joined lines. (Singapore built a separate circumferential, the Circle Line.) Arguably, the RER C falls into this category too, except the connection between the lines is too inner.
Reason 3: a half-formed circumferential
Hong Kong’s Kwun Tung Line is circumferential in the sense that it doesn’t serve Hong Kong Island, just Kowloon; partially because of water, it is C-shaped. New York’s G route used to be in this category back when it ran to Forest Hills, but in 2001 it was truncated to Court Square and became linear. Other lines in this category are hypothetical: if Paris’s Metro 2 and 6 count as C-shaped, then they fall into this category; Boston’s busiest bus, Line 66, is vaguely C-shaped, acting as a circumferential in the southwestern arc from Harvard to Dudley; and if New York builds Triboro RX then it will fall into this category, too. In this case, usually another reason, or a pure ridership concern, is what prevents completing the line as a full circle, but the line is configured to be useful for interchanges. The Kwun Tung Line is useful for end-to-end trips, but the other hypothetical cases aren’t: Triboro RX would be useful for short trips, but to get from the Bronx to southern Brooklyn, the D is much faster.
Reason 4: administrative boundaries
In regions without much intergovernmental cooperation, administrative boundaries can be as sharp as coastlines. Everything proceeds as in reason #1, but this time the inefficiency is entirely preventable. This specifically affects New York and SEPTA Regional Rail. Morally, New York’s north-south lines should connect the Bronx with Brooklyn and the east-west lines should connect Queens with New Jersey. But because New Jersey is administratively separate, the Queens lines loop back into Brooklyn, creating some awkward shapes on the F, the R, and especially the M both before and after its recent combination with the V. (Some Bronx-Brooklyn lines are also awkwardly shaped, but this is because of water). Likewise, SEPTA Regional Rail barely goes into New Jersey, and only in Trenton; PATCO, serving Camden, is separate, and as a result, while the system had the R# designations, the R5 and R6 were C-shaped and the R7 and R8 self-intersected, helping ensure there was not much suburb-to-suburb ridership.
Reason 5: aberrations
In some cases, such as the Marunouchi Line or Singapore’s self-intersecting Downtown Line, there’s no apparent reason, and in that case the two branches combine to form a C-shaped line for essentially random reasons. Maybe the ideal route through city center is one that connects two branches in the same direction; maybe there is more demand to one direction than to the other.
Of the above five reasons, it is reason 4 that is the most angering. Jersey City and the hill cities to its north have as long a history of ferry-oriented New York suburbanization as Brooklyn. But because of administrative reasons, they never got as much rapid transit, stunting their development. New York’s subway plans never really made any use of the Hudson Tubes, and even the unrealized plans for a North Jersey subway network made surprisingly little use of existing infrastructure. The result: 12 km out of Manhattan, at the same distance as Flushing, New Jersey only has Bogota, Rutherford, and Hackensack; 20 km out, at the same distance as still fairly dense Cambria Heights, New Jersey has Paramus and Montclair.
It’s of course too late for New York to do things right, but for a city just beginning to build a subway network, it’s important to make sure that lines are straight and hit developing suburbs in all directions, so that they can develop as high-density transit-oriented communities, and not as low-density auto-oriented ones.