Island platform

An island platform (also known as center platform or centre 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.

Advantages and tradeoffs

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, 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 plaform can impose undesirable speed limits, such as at Belmore, Turrumurra and Helensburgh in Sydney.

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. Whilst 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.

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. Another layout is to use two side platforms for local services with an island in between for fast services.

Island platforms are popular in the modern railway world for several reasons. Besides 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. As commuters make up a large number of railway passengers, this tends to mean that most people are using trains in one direction in the morning and most of them are using trains in the other direction in the evening. With two side platforms, this means that one platform is crowded while the other is deserted. An island platform prevents this as the same large platform is used for trains in both ways.

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 either platforms without using a bridge, subway or track crossing.

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. 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 without having to move the station platforms.

References

* [http://www.railway-technical.com/stations.shtml#Island-Platform Railway Technical Web Pages]