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general::
the subway
American English the railway system that runs under the ground below a big city. British people use the underground when talking about the railway system in London, and many systems outside the US:
• the New York subway • We got on the next subway train.
the underground
British English the railway system that runs under the ground below a big city:
• the London underground • an underground map • It’s quickest to travel by underground.
the tube
British English informal the railway system that runs under London:
• We took the tube to Trafalgar Square. • a crowded tube train
the metro
used especially to talk about the railway system that runs under Paris. Also used about some other city's systems, for example Moscow, Washington DC and Newcastle:
• the Paris Metro • Moscow’s metro stations
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