I’ve written before about some problems of metro network design in large cities. In brief, it’s important to maximize network effects in a multi-line system, which means offering plenty of transfers between lines. The perfect network should have every pair of lines intersecting in the center with a transfer, with possible additional intersections outside the center, again with transfers. In practice, it never works quite this way. There are always compromises, based on particular historical and geographical details of city layout. But probably the single biggest contributor to the issue is transfer capacity. This issue also has independent interest, but the two worst examples I know of involve the central transfer points of London and Paris, where many lines converge.
For a start, it’s worth asking why even have multiple stations. Why not just build a perfect star-shaped system? Two-line subway networks usually just cross once in city center. Three-line networks can intersect at one point (as in Stockholm and the first three lines of Moscow), but more commonly they intersect in a triangle of three city center stations. Central transfer points go way beyond three lines, though: Otemachi has five subway lines; Tokyo Station has a subway line and six independent JR East commuter lines; Chatelet-Les Halles has five Metro lines and two and a half RER lines; Bank and Monument together have four Underground lines and the Docklands Light Railway; Times Square has five subway lines, three of which are four-track. Why not just add more lines to the same central station? There are three distinct answers.
Transit networks aren’t just about connecting large neighborhoods (“Upper West Side”) to a nebulously defined city center. They’re about specific connections. City centers are larger than a single subway stop, and much larger than a single subway stop in any city that has any business building four or more subway lines. In Stockholm, where three lines is about right, the CBD extends about two stops heading north and east of T-Centralen. New York, Paris, Tokyo, and London all have CBDs several square kilometers in area, so it makes sense to route lines in such a way that it’s easy to reach many points within the CBD from all directions.
Concretely, take Times Square and Grand Central. It’s useful to serve both of them on multiple subway lines, but a north-south line can only serve one. Thus, the 4/5/6 serve Grand Central, and the A/C/E, 1/2/3, and N/Q/R/W serve Times Square. The same process repeats itself at a number of nodes within Midtown, and within CBDs of other large cities.
Independently of the value of having extensive service in multiple directions from multiple points in the CBD, there is the cost of bringing lines together. In large cities, the biggest source of missed connections to begin with is that the street available for line 6 may happen to pass right between two widely-spaced stops on line 1, which never had a stop at this street because line 6 was not in the planning stages yet.
For the same reason, urban street networks make it difficult to serve one point from more than a few directions. Even lines bored deep under the surface, without regard for the street network, would find it difficult to go on level -9, beneath eight older lines. The stations with the largest number of independent lines all have tricks to make this work. Times Square has three north-south subways that don’t physically intersect (the 1/2/3 is always to the west of the N/Q/R/W and the A/C/E well to the west of both), a stub-ending east-west subway (the shuttle), and one deep-bored east-west subway (the 7). At Tokyo Station four of the six commuter lines are elevated at the same level. At Otemachi the lines form a square, with one side consisting of two lines that were built together at the same time. Chatelet-Les Halles has platforms that do not intersect, and the most difficult retrofit, the addition of the RER, was a massive excavation project that cost billions of euros.
The same construction difficulties are also relevant to small transfer stations. In Paris, transfer stations try to avoid superimposing one line’s platforms on top of the other, just because it’s hard to build. As a result, transfers often involve long walks; transfers at Chatelet are particularly labyrinthine.
The biggest problem is not coverage, and only partly related to construction difficulties. At the busiest stations, pedestrian circulation between platforms can be a challenge. London Reconnections has a four–part series about Bank, where three deep-level Underground lines meet, all having built in the late 1890s, when expected ridership was far lower than it is today. Circulation is so obstructed that at rush hour TfL occasionally has to close the station for safety reasons, or else passengers would fall onto the tracks. Retrofitting the station with additional connections between lines as well as from the platforms to the street has been a daunting task, since the most logical places for escalators to one line would often pass through the platforms of another.
At Chatelet-Les Halles, the same problem occurs, if not so acutely that trains need to skip the station at rush hour. The passageways between the Metro platforms and between the Metro and the RER are long and narrow, and barely adequate to handle the large volume of passengers.
The simplest way to prevent this problem from occurring at a particular station is to make sure to design enough room for the transfer. The simplest way to do that, in turn, is to ensure transfers are cross-platform. The RER has such cross-platform transfers at the station, pairing westbound RER A trains with northbound RER B trains and eastbound RER A trains with southbound RER B trains. But even the wrong-way RER A-B transfers and the transfers involving the RER D are fine: the station’s extreme cost paid for a full-length, full-width mezzanine. The London Underground, too, retrofitted these onto some older lines when it built the Victoria line, which has cross-platform transfers at such key stations as Oxford Circus (the busiest in London not counting mainline stations – Bank is only the second busiest) and Euston.
However, cross-platform transfers connect two lines. I know of one place where they connect three: Jamaica Station, where one track in each direction has platforms on both sides, and passengers on the trains on the opposite sides of these platforms are sometimes told to walk through the train to transfer. This is a unique feature of regional rail, with its timed connections; subways with a train every 2-3 minutes can’t realistically time the connections, and without timing the connections, passengers are better off walking up and down to the other platform than waiting for a train to come in for a purely horizontal transfer.
The need for coordinated planning
The ultimate problem with using more cross-platform transfers if that they require a great deal of foresight. Retrofitting them is not always possible, and costs money in modification of existing stations. Hong Kong, Singapore, and Taipei all use these transfers extensively, but only on lines that were planned together, such as the first two lines in Singapore; Singapore’s newer lines have long (though spacious) transfer corridors, and Hong Kong’s lines inherited from the original MTR and from mainline rail have poor transfers.
With relatively limited opportunities to have high-capacity, high-quality transfers, it’s no wonder that most cities that build rapid transit try to avoid four- and five-way transfers when possible. Complex transfers like this can arise by accident, over several layers of planning – in the case of Paris, Line 11 was planned and built a generation later than Lines 1, 4, and 7; the RER was built a generation later than Line 1; and Line 14 was built a generation later than the RER, and indeed was designed as a relief line to the RER A.
Ultimately, the best way to prevent a situation like Chatelet or Bank from occurring is to know in advance where every line will go. However, this is necessarily a hard task. In the 1890s, London was a city of 6 million, with a large number of poor people living in overcrowded condition in East and South London; a planner could guess how the city would grow and suburbanize in the 20th century but would not be able to predict this with any certainty. Paris, the capital of a then-poorer and far less industrialized country, has grown even more tremendously – in 1901 Ile-de-France had 4.7 million people, not all living in the built-up area of the capital.
In very large third-world cities, the task of predicting future growth is somewhat easier, but only because they’re already very large and have informal transit pointing the way to the major corridors. I can draw a semi-serious Lagos metro proposal based on the city’s urban layout today and expect much of its future growth to come from increasing building heights so that the same density can be accommodated with less overcrowding, but I can’t meaningfully say which future areas will become hotspots that must be served from all directions or how far the suburban sprawl will go.