Island platform

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An island platform on a railway is where a single platform lies between two tracks, serving both of them. Usually, the two tracks are on the same line, running in opposite directions. One station may have two island platforms in a four-track express configuration; in this case each platform may serve trains in one direction, with local and express trains stopping on opposite sides of a single platform.

[edit] Advantages and Tradeoffs

An example of an Island platform

Island platforms generally have a lower construction cost and 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. 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 about 4 meters (13 ft). If the island platform is 6 meters (20 ft) wide, the tracks have to slew out by the same distance.

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.

Island platforms are popular in the modern railway world for several reasons. One is their lower construction cost. Island platforms also 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. Passenger convenience is another significant consideration. Generally, even able-bodied passengers dislike climbing steps to pass between platforms, and in some areas subways 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.

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. Many of the stations on the Great Central Railway (now closed) were constructed in this form.