A cross-platform interchange is a type of interchange between different lines in a metro system. The term originates with the London Underground; such layouts exist in other networks but are not commonly so named. In the United States, it is often referred to as simply a "transfer".
It occurs in a system with island platforms, with a single platform in between the two directions of travel, or two side platforms between the tracks, connected by level corridors. Passengers do not need to move to another platform level for transfer, thus increasing the efficiency of commuting. Building a cross-platform interchange may be costly due to the complexity of railtrack alignment, especially so if the railway operators insistently arrange their track without flat crossings. A common two-station cross-platform interchange configuration consists of two similar directions of two different lines sharing an island platform in one station, and two opposite directions of two different lines sharing an island platform in the other station.
The New York City Subway is primarily made up of four-track lines with local and express service. Cross-platform interchanges in numerous locations throughout the city generally allow for cross-platform transfer between express train and local train service. Express trains run on the inner tracks and bypass local-only stations, while local trains run on the outer tracks and stop at every station. Express stations typically have island platforms between the express and local tracks allowing passengers to quickly switch trains. In addition to the very common express-local interchanges, New York also has several cross-platform interchanges between lines that do not share a four-track right-of-way. A notable example of this is the Queensboro Plaza station with same-directional cross-platform transfer between the IRT Flushing Line and BMT Astoria Line.
Three adjacent stations of the BART system, namely Oakland 12th Street, Oakland 19th Street and MacArthur stations, feature cross-platform interchange between the Bay Point-San Francisco Airport/Millbrae (yellow) line and the Richmond-Fremont (orange) line. The trains are so timed that the trains bound for Bay Point and for Richmond arrive at the same time. On the other hand, only MacArthur station has enough platforms to enable such "timed" transfer between trains bound for San Francisco Airport/Millbrae and for Fremont.
In London's deep-level tube network, these usually occur in pairs for both directions of two lines. This allows for extremely quick and convenient interchange. The effect is that the two lines, despite having completely separate operation, can be treated by passengers as branches of a single network. Examples include: Finchley Road (Metropolitan and Jubilee lines at surface level) and Mile End (District and Central lines at sub-surface level). For the deep-level interchanges the term "same level interchange" is sometimes preferred as there is invariably an intermediate circulating area between the platforms. Convenient same level interchanges feature at various Victoria line stations and also at Baker Street (between Bakerloo and Jubilee lines).
In the Mass Transit Railway (MTR) system of Hong Kong, cross-platform interchange stations first appeared at Mong Kok and Prince Edward stations when Tsuen Wan Line was opened. The interchange is two stations long, with inverse cross-platform interchange assigned to Prince Edward and same-direction interchange to Mong Kok as large volumes of traffic had been anticipated in both modes. Popular with passengers, this design was repeated on Admiralty Station when the Island Line was opened to deal with the heavy wrong-direction interchange anticipated. Years later, another interchange spanning Tiu Keng Leng Station (same direction) and Yau Tong Station (wrong direction) was built to the same configuration as Mong Kok and Prince Edward (see diagram). The interchange from Ma On Shan Line to East Rail Line is so designed that trains on the Ma On Shan Line observe right-hand traffic rules, unlike other road or rail traffic in Hong Kong. This cross-platform arrangement between southbound trains of the two lines creates convenience for most morning commutes. Another configuration found in Lai King Station makes commute in both directions more convenient by aligning both tracks leading towards city next to each other on one level, and both tracks leading away from city next to each other on the other level.
The Mass Rapid Transit system in Singapore has a similar two-station transfer arrangement to allow quick transfers between North South and East West lines. Both City Hall MRT Station and Raffles Place MRT Station have double underground island platforms stacked atop the other, allowing commuters to switch trains to a different line by walking across the same platform at the appropriate station. Jurong East MRT Station has a less complicated arrangement, with the terminating rail for the North South line aligned between that of the East West line, allowing commuters to alight and board simultaneously on either side.
Two transfer stations in Montreal feature cross-platform interchange. At Lionel-Groulx station, the upper platforms serve Henri-Bourassa or Montmorency (orange line 2) and Honoré-Beaugrand (green line 1) trains, entering downtown; the lower platforms serve Côte-Vertu (orange line 2) and Angrignon (green line 1) trains, leaving downtown. Since most transferring passengers are either entering or leaving downtown, most transfers at this station are cross-platform.
At Snowdon station, however, the outbound orange line platforms are at the same level as the blue line terminal platform, with the inbound orange line platforms linked to the blue line departure platform, reducing efficiency. This is explained because the blue line was originally planned to be continued west of the station, in which case this arrangement would have provided the same benefit as the arrangement at Lionel-Groulx.
A cross-platform interchange is a type of interchange between different lines in a railway system.