Island platform

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 and can provide for services in both directions from a single platform requiring only one set of supporting services (toilets, ticket offices, kiosks). 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. An alternative arrangement is to position side platforms on either side of the tracks.

Layout
Station with 2 tracks and one island platform

Island platforms are popular in the modern railway world for several reasons. Island platforms allow facilities such as escalators, elevators, shops, toilets and waiting rooms to be shared between both tracks rather than being duplicated or present only on one side. On commuter rail lines, passengers tend to use trains in one direction in the morning and the other direction in the evening. With two side platforms, one platform becomes crowded while the other is deserted. An island platform prevents this as the same large platform is used for trains travelling in either direction. This also reduces the cost of maintenance, because rather than having to maintain two separate side platforms, railway operators only have to maintain one island platform.

Passenger convenience is another significant consideration. Generally, even able-bodied passengers dislike climbing steps to pass between platforms, and in some areas subways (i.e. pedestrian walkways) under the railway line may also pose vandalism and security problems. A growing consideration is the requirement for wheelchair accessible stations. An island platform makes it easier for wheelchair users and the infirm to change services, but it means that on a station at ground level, it is impossible to reach the platform without using a bridge, underpass, or track crossing. On the other hand, island platform subway stations allow passengers to use any station entrance, and it eliminates the need for some signage, as well as eliminating the need to construct a crossover or crossunder between two platforms.

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 require less space than side platforms, a pair of separate platforms with the tracks running between them. However, island platforms may become overcrowded, especially at busy stations, and this can lead to safety issues such as Clapham Common (see image) and Angel (now rebuilt) on the London Underground, or else the curves at either end of an island platform can impose undesirable speed limits, such as at Belmore, Turrumurra and in Melbourne.

Additionally, the need for the tracks to diverge around the center platform requires extra width along the right-of-way on each approach to the station, especially on high-speed lines. Track centers vary from rail systems throughout the world but are normally 3 to 5 meters (10 to 16 ft). If the island platform is 6 meters (20 ft) wide, the tracks have to slew out by the same distance. While this is not a problem on a new line that is being constructed, it makes it impossible to build a new station on an existing line without altering the tracks. In addition, a single island platform 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 uses 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 Grand Army Plaza in the New York City Subway). 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, as at 34th Street – Penn Station or at 34th Street – Penn Station on the New York City Subway, uses two side platforms for local services with an island in between for fast services.

Examples
Many of the stations on the Great Central Railway (now closed) were constructed in this form. This was because the line was planned to connect to a Channel Tunnel. If this happened, the lines would need to be compatible with continental loading gauge, and this would mean it would be easy to change the line to a larger gauge, by moving the track away from the platform to allow the wider bodied continental rolling stock to pass freely while leaving the platform area untouched.

Unused sides of island platforms
Sometimes when the track on one side of the platform is unused by passenger trains, that side may be fenced off. Examples include Hurlstone Park and Lewisham.

In New York City's subway system, unused sides can be found at Lexington Avenue – 63rd Street and at Bowling Green. (The former has an island platform for an unused cross-platform interchange; the latter has a side platform in addition to its island platform, making one side of the island platform unnecessary.) In Jersey City, the Newport PATH station has the same configuration as Bowling Green—one side platform and one island platform.