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Urban rail transit

Urban rail transit is an all-encompassing term for various types of local rail systems serving urban or older suburban areas. The vast majority of modern urban rail vehicles run on electricity. The set of urban rail systems can be roughly subdivided into two categories, which sometimes overlap, causing some systems or lines to have aspects of each.

  • Whatever we decide to call tram/streetcar refers to a system that runs mainly or completely along streets, with low capacity and frequent stops. Loading it done at street or curb level (but low-floor vehicles may be used). These can be called trams, streetcars and trolleys, and include longer-distance interurbans.
  • Light rail is a relatively new class, as an outgrowth of whatever we decide to call tram/streetcar. Speeds are usually higher, and articulated vehicles may be used to increase capacity. Note that some systems called light rail have most or all of the characteristics of rapid transit (see below) and may be better placed in that category, while others are essentially trams referred to as light rail for political reasons.
  • Rapid transit typically runs grade-separated from all intersecting roads, in tunnels or on elevated structures, or in open cuts in outlying areas. Trains typically run faster than light rail and stops are less frequent. Platforms are usually level with the typically high floors of the trains, and trains can reach ten or more cars in length (with multiple-unit operation), providing more capacity than light rail at higher headways. Electricity is usually provided by a third rail, though overhead wires are sometimes used, particularly by systems such as the Tyne and Wear Metro which make extensive use of surface-level track. Fares are collected before boarding, and usually proof of payment is required to even enter a station's platforms. Systems of this type can be called metros, subways, undergrounds, elevated railways, or sometimes heavy rail, though this term is more commonly used to refer to mainline and regional railways (see below).

Terms typically used for one type of system are sometimes used for the other. For example, Boston's Green Line is referred to as a subway, despite having street-running portions. The Docklands Light Railway in London is a predominantly-elevated system which provides a metro-style service with more in common with the rapid transit definition above than that of light rail; it is so named to distinguish it from the London Underground, which uses longer trains of heavier vehicles to provide more frequent service.

Many cities use names such as subway and elevated railway to describe their entire systems, even when they combine both methods of operation; slightly less than half of the London Underground's tracks, for example, are actually underground, while the Chicago El and Vancouver SkyTrain incorporate some tunnelled tracks.

Other types of passenger rail include the following:

  • Funiculars are inclined railways that carry passengers up and down steep slopes.
  • Regional rail or commuter rail runs on trackage often shared with intercity rail and freight trains, typically serving newer suburbs and rural areas. Commuter rail trains are typically built to higher standards, as they run at higher speeds are at risk of more severe crashes. This distinguishes commuter rail from interurbans, which use light-rail vehicles on tracks through lower density areas.

A bus shares many characteristics with light rail, but does not run on rails. Trolleybuses are buses that run off overhead wires. Railbuses, vehicles that can travel both on rails and on roads, have been tried experimentally, but are not in common use. The term bus rapid transit is used to refer to various methods of providing faster bus services, but the systems which use it are usually more equivalent to light rail than to rapid transit.

For terminology, see passenger rail terminology.

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