A rapid transit line is a rail vehicle transportation corridor, usually limited to a single urban area, with a dedicated right-of-way separate from road vehicle corridors. These include metro lines and subways, and may be elevated, underground, or at ground level. They generally do not connect to an intercity rail network.

The dedicated route can lie above, below, or at ground level. Its separation from road-vehicle corridors is not a requirement that it be underground.

The service is generally limited to one urban area and separate from intercity rail. A tramway more commonly follows the city street network, with possible dedicated sections.