What is the meaning of Subway?

An underground railway, especially for mass transit of people in urban areas.

A train that runs on such an underground railway.

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

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

An underground route for pipes, sewers, etc.

To travel by underground railway.

subway

Source: wiktionary.org