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 (lifts, escalators, 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.

Station with two 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.

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Wikipedia