A rapid transit, underground, subway, tube, elevated, or metro(politan) system is a railway system, usually in an urban area, with a high capacity and frequency of service, and grade separation from other traffic. There is no one term that all English speakers use for rapid transit or metro systems. This is a reflection on national and regional usage and differing definitions of what characteristics are essential for these systems. In larger metropolitan areas the underground system may extend only to the limits of the central city, or to its inner ring of suburbs, with trains making relatively frequent station stops. The outer suburbs may then be reached by a separate commuter, suburban or regional rail network, where more widely spaced stations allow higher train speeds. In some cases, the rapid transit system runs to the suburbs and effectively functions as a regional rail service as well. Where there are separate systems, the rapid transit system is typically a self-contained service with its own dedicated tracks and stations and is often technologically incompatible with other railways. Suburban rail services, on the other hand, often share tracks and stations with long-distance trains and are subject to the same standards and regulations.
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