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
An underground passage, subway, tunnel.
A voyage, especially underwater.
A road, track, path, or street for going under another way or obstacle; an underpass.
In motion, in progress; being done or carried out; on a journey.