Considerations
A metro system is defined as an urban, electric passenger transport system with high capacity and high frequency of service, which is totally independent from other traffic, road or pedestrians. The terms heavy rail (mainly in North America) and heavy urban rail often have similar definitions.
The dividing line between metro and other modes of public transport, such as light rail and commuter rail, is not always clear. 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, metro systems are primarily used for transport within a city, and have higher service frequency, typically not more than 10 minutes between trains during normal daytime service. 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 lines.
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 match the technical level and service standards of metro systems, but reach far out of the city and are commonly known or better described as suburban, regional or commuter rail. These are not included. Neither are monorail and funicular systems, or people movers, such as amusement park, ski resort and airport transportation systems.
Read more about this topic: List Of Metro Systems