Island Platform - Advantages and Tradeoffs

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 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. 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. A rarer layout, as at Atlantic Avenue station on the New York City Subway, uses two side platforms for local services with an island in between for fast services.