A rapid transit rail transport system, or a train in such systems, generally underground and serving a metropolitan area.
An urban rapid transit rail transport system, normally with lighter track and light trains (i.e. an S-Bahn or similar), or a train in such systems.
A metropolitan area.
metropolitan
A commuter rail system that combines aspects of an interurban railway with those of a subway, using a menagerie of pathways such as viaduct, tunnel, beam, cut and cover, trench, at grade, and others.