<P> This list of metro systems includes electrified rapid transit train systems worldwide . In some cases, metro systems are referred to as subways, U-Bahns 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 first 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 . </P> <P> 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". </P>

Who has the largest subway system in the world
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