Urban rail transit

Urban rail transit is an all-encompassing term for various types of local rail systems providing passenger service within and around urban or suburban areas. The set of urban rail systems can be roughly subdivided into the following categories, which sometimes overlap because some systems or lines have aspects of each.

A tram, streetcar or trolley system is a rail-based transit system that runs mainly or completely along streets (i.e. with street running), with relatively low capacity and frequent stops. Passengers usually board at street- or curb-level, although low-floor trams may allow level boarding. Longer-distance lines are called interurbans or radial railways. Few interurbans remain, most having been upgraded to commuter rail or light rail or abandoned.

The term "tram" is used most parts of the world. In North America, these systems are referred to as "streetcar" or "trolley" systems; in Germany, such systems are called "Straßenbahn" which literally translates as "street train" or "street railway".

A light rail system is a rail-based transit system that has higher capacity and speed than a tram, usually by operation in an exclusive right-of-way separated from automobile traffic, but which is not fully grade-separated from other traffic like rapid transit is. Light rail also generally operates with multiple unit trains rather than as single tramcars. It emerged as an evolution of trams/streetcars. Light rail systems vary significantly in terms of speed and capacity. They range from slightly improved tram systems to systems that are essentially rapid transit but with some level crossings.

The term "light rail" is the most common term used, though German systems are called "Stadtbahn" (which translates as "city railway").

A rapid transit, underground, subway, tube, elevated, metro or Mass Rapid Transit (MRT) system is a railway—usually in an urban area—with high passenger capacities and frequency of service, and (usually) full grade separation from other traffic (including other rail traffic). It is often known as "heavy rail" to distinguish it from light rail and bus rapid transit.

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