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-Bahn or undergrounds. , 192 cities in 61 countries have a metro system. 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 Shanghai Metro is the world's longest metro network at and also has the highest annual ridership at 2.83 billion trips. The New York City Subway has the greatest number of stations with 472. The country with the most metro systems is China, with 47 in operation. 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