Definitions
1.NounAn underground railway, especially for mass transit of people in urban areas."He is going to Westminster by subway."
2.NounA train that runs on such an underground railway.
3.NounA rapid transit system, regardless of the elevation of its right of way; a metro system.
4.NounAn underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
5.NounAn underground route for pipes, sewers, etc.
6.NounAn underground route for creatures (e.g. bats).
7.VerbTo travel by underground railway.