An island platform (also center platform, centre platform) is a station layout arrangement where a single platform is positioned between two tracks within a railway station, tram stop or transitway interchange. Island platforms are popular on twin-track routes due to economic and land use reasons. They are also useful within larger stations where local and express services for the same direction of travel can be provided from opposite sides of the same platform thereby simplifying transfers between the two tracks. An alternative arrangement is to position side platforms on either side of the tracks. The historical use of island platforms depends greatly upon the location. In the United Kingdom the use of island platforms is relatively common when the railway line is in a cutting or raised on an embankment, as this makes it easier to provide access to the platform without walking across the tracks.
Advantages and tradeoffs
Island platforms are necessary for any station with many through platforms. There are also advantages to building small two-track stations with a single island platform instead of two side platforms. Island platforms allow facilities such as shops, toilets and waiting rooms to be shared between both tracks rather than being duplicated or present only on one side. An island platform makes it easier for disabled travellers to change services between tracks or access facilities. If the tracks are above or below the entrance level, the station needs only one staircase and (if disabled accessibility is necessary) one elevator or ramp to allow access to the platforms. If the tracks are at the same level as the entrance, this instead creates a disadvantage; a side platform arrangement allows one platform to be adjacent to the entrance, whereas an island platform arrangement requires both tracks to be accessed by a bridge or underpass.
If an island platform is not wide enough to cope with passenger numbers, overcrowding can be a problem. Examples of stations where a narrow island platform has caused safety issues include Clapham Common and Angel (rebuilt in 1992) on the London Underground, Union (rebuilt in 2014) on the Toronto subway, and on the Osaka Municipal Subway.
An island platform requires the tracks to diverge around the center platform, and extra width is required along the right-of-way on each approach to the station, especially on high-speed lines. Track centers vary for rail systems throughout the world but are normally . If the island platform is wide, the tracks must slew out by the same distance. While this requirement is not a problem on a new line under construction, it makes building a new station on an existing line impossible without altering the tracks. A single island platform also makes it quite difficult to have through tracks (used by trains that do not stop at that station), which are usually between the local tracks (where the island would be).
A common configuration in busy locations on high speed lines is a pair of island platforms, with slower trains diverging from the main line (or using a separate level on the railway's right-of-way) so that the main line tracks remain straight. High-speed trains can therefore pass straight through the station, while slow trains pass around the platforms (such as at Kent House in London). This arrangement also allows the station to serve as a point where slow trains can be passed by faster trains. A variation at some stations is to have the slow and fast pairs of tracks each served by island platforms (as is common on the New York City Subway; the Broad Street Line of Philadelphia; and the Chicago Transit Authority's Red and Purple lines).
A rarer layout, present at Mets-Willets Point on the IRT Flushing Line, 34th Street – Penn Station on the IRT Seventh Avenue Line and 34th Street – Penn Station on the IND Eighth Avenue Line of the New York City Subway, uses two side platforms for local services with an island in between for express services. The purpose of this atypical design was to reduce unnecessary passenger congestion at a station with a high volume of passengers. Since the IRT Seventh Avenue Line and IND Eighth Avenue Line have adjacent express stations at 42nd Street, passengers can make their transfers from local to express trains there, leaving more space available for passengers utilizing intercity rail at Pennsylvania Station. The Willets Point Boulevard station was renovated to accommodate the high volume of passengers coming to the 1939 World's Fair.