I did a complex Patreon poll about series to write about. In the poll about options for transit network design the winning entry was difficult urban geography, covered here and here; the runner-up was cross-platform transfers.
Subway users have usually had the experience of connecting at a central station so labyrinthine they either were lost or had to walk long distances just to get to their onward train. Parisians know to avoid Chatelet and New Yorkers know to avoid Times Square. It’s not just an issue for big cities: every metro system I remember using with more than one line has such stations, such as T-Centralen in Stockholm, Waterfront in Vancouver, and Dhoby Ghaut in Singapore. To prevent such connections from deterring passengers, some cities have invested in cross-platform interchanges, which permit people to transfer with so little hassle that in some ridership models, such as New York’s, they are treated as zero-penalty, or equivalent to not having to transfer at all.
Unfortunately, improving the transfer experience is never as easy as decreeing that all interchanges be cross-platform. While these connections are always better for passengers than the alternative, they are not always feasible, and even when feasible, they are sometimes too expensive.
Cross-platform transfer to wherest?
Consider the following two-line subway interchange:
A cross-platform transfer involves constructing the station in the center so that the north-south and east-west lines have platforms stacked one on top of the other, with each east-west track facing a north-south track at the same platform. The problem: do eastbound trains pair up with northbound ones and westbound trains with southbound ones, or the other way around?
In some cases, there is an easy answer. If two rail lines heading in the same general direction happen to cross, then this provides a natural pairing. For example, the Atlantic Branch and Main Line of the LIRR meet at Jamaica Station, where the cross-platform transfer pairs westbound with westbound trains and eastbound with eastbound trains. In Vienna, this situation occurs where U4 and U6 intersect: there is a clear inbound direction on both lines and a clear outbound lines, so inbound pairs with inbound and outbound with outbound.
However, in most cases, the transfer is within city center, and there is no obvious pairing. In that case, there are two options.
Near-cross platform transfer
Some transfers are nearly cross-platform. That is to say, they have trains on two levels, with easy vertical circulation letting people connect between all four directions. In Berlin, there is such a transfer at Mehringdamm between U6 and U7 – and in the evening, when trains come every 10 minutes, they are scheduled to offer a four-way timed interchange, waiting for connecting passengers even across a level change.
Multi-station transfer complex
Singapore, Stockholm, and Hong Kong all offer cross-platform transfers in multiple directions by interweaving two lines for two or three consecutive stations. The three-station variant is as in the following diagram:
At the two outer transfer stations, the cross-platform connections are wrong-way relative to the shared trunk corridor: eastbound pairs with northbound, westbound pairs with southbound. At the middle station, connections are right-way: eastbound pairs with southbound, westbound pairs with northbound.
Of note, the shared trunk has four tracks and no track sharing between the two different subways. I’ve proposed this for the North-South Rail Link. The reason three stations are needed for this and not two is that with only two stations, passengers would have to backtrack in one pairing. Nonetheless, backtracking is common: Stockholm has three stations for the transfer between the Green and Red Lines but only the northern one is set up for wrong-way transfers, so passengers connecting wrong-way in the south have to backtrack, and Singapore has two stations between the East-West and North-South Lines, since one of the pairings, west-to-south, is uncommon as the North-South Line extends just one station south of the transfer.
Why are they not more widespread?
The inconvenience of Parisian transfers is a general fact, and not just at Chatelet. Two lines that meet usually meet at right angles, and the platforms form a right angle rather than a plus sign, so passengers have to be at one end of the train to have easy access to the connecting platforms. The reason for this is that Paris built the Metro cut-and-cover, and there was no space to reorient lines to have cross-platform transfers.
In contrast, both Stockholm and Singapore had more flexibility to work with. Singapore deep-bored the MRT for reasons of civil defense, contributing to its recent high construction costs; the tradeoff is that deep boring does permit more flexibility underneath narrow streets, which all streets are compared with the footprint of a cross-platform interchange. Stockholm used a mixture of construction methods, but the four-track trunk carrying the Green and Red Lines is above-ground in the Old City but was built with a sunk caisson at T-Centralen.
In London, similarly, there are cross-platform transfers, involving the Victoria line. It was built in the 1960s around older infrastructure, but at a few spots in Central London, the tubes were built close enough to old lines to permit cross-platform interchange in one direction (northbound-to-northbound, southbound-to-southbound). In contrast, the surface network, constrained by land availability, does not feature easy interchanges.
While deep boring makes cross-platform transfers easier, either can exist without the other. If I understand this correctly, U6 was built cut-and-cover. There were even weaves on the IND in New York, but they were expensive. Moreover, when two lines are built under a wide street with two branching streets, rather than on something like a grid (or even Paris’s street network, which is gridded at key places like where M4 runs under Sevastopol), cut-and-cover construction can produce a cross-platform transfer. Conversely, such transfers do not exist in all-bored Moscow and are rare in London.
The importance of planning coordination
Ultimately, cross-platform transfers boil down to coordinated planning. Some cities can’t build them even with coordination – Paris is a good example – but absent coordination, they will not appear no matter how good the geography is. Stockholm, Berlin, Vienna, Singapore, and Hong Kong are all examples of centrally planned metro networks, without the haphazard additions of New York (which was centrally planned on three separate occasions) or London (where the early lines were built privately).
Even with coordination, it is not guaranteed cross-platform transfers will appear, as in Moscow. Planners must know in advance which lines they will build, but they must also care enough about providing a convenient transfer experience. This was not obvious when Moscow began building its metro, and regrettably is still not obvious today, even though the benefits are considerable. But planners should have the foresight to design these transfers when possible in order to reduce passenger trip times; ultimately it is unlikely to cost more than providing the same improvements in trip times through faster trains.