Definitions
1.nounAn underground railway, especially for mass transit of people in urban areas.
2.nounA train that runs on such an underground railway.
3.nounA rapid transit system, regardless of the elevation of its right of way.
4.nounAn underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
5.nounAn underground route for pipes, sewers, etc.
6.verbTo travel by underground railway.