noun
(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.
(British) An underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
An underground route for pipes, sewers, etc.
An underground route for creatures (e.g. bats).
verb
(intransitive, US, informal) To travel by underground railway.