I see "metro" defined as "subway" = "an underground metropolitan railway" elsewhere in these comments. Any American visitors to the UK might want to know that this is not what the local mean by "subway".
a subway is an underground pedestrian passage to enable people to cross a busy city road. A metro is an integrated urban transport system organised around buses and trains. Except in London, of course, where Transport for London is the body that looks after the road system, the buses and the urban trains. The trains are both underground and overground, but are commonly called "the underground" (never the metro) or "the tube". I had occasion to help out a very nonplussed American in London some years ago. He was searching the underground map for "Lie-sester-shyer" Square. I directed him to Leicester Square (pronounced Lester). He thanked me and toddled off, shaking his head at "these crazy Brits".