地铁(dì tiě) - underground, subway in Mandarin

地铁 (dìtiě) is the "underground" or "subway", the train system that runs beneath a city. 地 (dì) means "ground, earth" and 铁 (tiě) means "iron, metal", so together they give "underground iron (road)", the same logic behind the English "underground" and the French "métro".

It is now the backbone of getting around any large Mandarin-speaking city: Beijing, Shanghai, Taipei and Hong Kong (地鐵 (dìtiě) in the traditional-script cities) all run huge networks, and 地铁站 (dìtiě zhàn), "underground station", is one of the first compounds any visitor learns.

British learners should note that Mandarin does not distinguish "underground" from "subway" or "metro" the way regional English does; 地铁 (dìtiě) covers all of them. Say 坐地铁 (zuò dìtiě), "to take the underground", using 坐 (zuò) rather than 骑 (qí), the same verb you would use for a bus or a train.