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.