(Canada, US, Scotland, South Africa) An underground railway, especially for mass transit of people in urban areas.

(Canada, US, Scotland, South Africa) A train that runs on such an underground railway.

(Canada, US) A rapid transit system, regardless of the elevation of its right of way; a metro system.

(Britain) An underground walkway, tunnel for pedestrians (called pedestrian underpass in US).

An underground route for pipes, sewers, etc.

(intransitive, US, informal) To travel by underground railway.