List of metro systems

This list of metro systems includes electrified rapid transit train systems worldwide. In some parts of the world, metro systems are referred to as subways, U-Bahnen or undergrounds. As of December 2017, 178 cities in 56 countries around the world host the approximately 180 metro systems that are listed here. The London Underground first opened as an "underground railway" in 1863 and its first electrified underground line opened in 1890, making it the world's oldest metro system. The metro system with the longest route length is the Shanghai Metro; the busiest one is the Beijing Subway; and the one with the most stations is the New York City Subway.

Considerations
The International Association of Public Transport (L'Union Internationale des Transports Publics, or UITP) defines metro systems as urban passenger transport systems, "operated on their own right of way and segregated from general road and pedestrian traffic". The terms Heavy rail (mainly in North America) and heavy urban rail are essentially synonymous with the term "metro". Heavy rail systems are also specifically defined as an "electric railway".

The dividing line between metro and other modes of public transport, such as light rail and commuter rail, is not always clear, and while UITP only makes distinctions between "metros" and "light rail", the American Public Transportation Association (APTA) and Federal Transit Administration (FTA) distinguish all three modes. A common way to distinguish metro from light rail is by their separation from other traffic. While light rail systems may share roads or have level crossings, a metro system runs, almost always, on a grade-separated exclusive right-of-way, with no access for pedestrians and other traffic. And in contrast to commuter rail or light rail, metro systems are primarily used for transport within a city, and have higher service frequencies and substantially higher passenger volume capacities. Furthermore, most metro systems do not share tracks with freight trains or inter-city rail services. It is however not relevant whether the system runs on steel wheels or rubber tyres, or if the power supply is from a third rail or overhead line.

The name of the system is not a criterion for inclusion or exclusion. Some cities use metro as a brand name for a transit line with no component of rapid transit whatsoever. Similarly, there are systems branded light rail that meet every criterion for being a rapid transit system. Some systems also incorporate light metro or light rail lines as part of the larger system under a common name. These are listed, but the light rail lines are not counted in the provided network data. Certain transit networks may match the service standards of metro systems, but reach far out of the city and are sometimes known as S-Bahn, suburban, regional or commuter rail. These are not included in this list. Neither are funicular systems, or people movers, such as amusement park, ski resort and airport transport systems.

This list counts metros separately when multiple metros in one city or metropolitan area have separate owners or operating companies. This list expressly does not aim at representing the size and scope of the total rapid transit network of a certain city or metropolitan area. The data of this list should not be used to infer the size of a city's, region's, or country's urban rail transit systems, or to establish a ranking.