It depends. In Washington DC we call it the metro; in New York City they call it the subway; in Boston they call it the T.
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In America metro is usually short for metropolitan area (a big city and its area of influence). SUMMARY (simplified) = the tube (or the underground) = the subway / the train.
In America the first metro was built in New York, and it was called SUBWAY because it was built mostly under the streets (sub= under / way= road), since Manhattan street plan is quite a regular grill and it was easy to do so. But most everywhere else they usually call it simply the TRAIN.
Each region has a name for their transportation system, so it varies. For New York, I have mostly seen subway, for DC it's metro, for Boston and Pittsburgh it's the T, for Chicago it's the L. In terms of strict dictionary definitions, both terms imply underground trains.
The system is operated by a subsidiary of the Metropolitan Transportation Authority (MTA). The subway system is usually just referred to as the trains. Locals say I can take the train to your place to generally mean that they take the subway. The subway is never referred to as the metro, underground, or tube.
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 list was topped by the reliable, safe and comfortable public transport in Berlin. 97 percent of Berliners raved about their city's transport network, with their metro U-Bahn offering travellers 175 stations across nine simple-to-navigate lines.
The New York Subway opened in 1904 and is the State's longest metro system and one of the world's largest underground lines. It has nearly 500 stations and a total of 660 miles of tracks (1,060 km).
The New York metropolitan area is the city and suburbs of New York City. It includes Long Island and the Mid- and Lower Hudson Valley in the state of New York. It also includes north and central New Jersey, three counties in western Connecticut and five counties in northeastern Pennsylvania.