Hitachi builds real metro rail cars used in city transit systems, and a version fits a large GTA 6 urban map with public transport. Fans expect it running the local rail lines, but Rockstar has not confirmed it or given it an in-game name. This is expectation based on the setting, nothing more.
There is no confirmed in-game name or way to drive this metro car freely. City trains historically follow fixed routes and are rarely controllable, and Rockstar has shown nothing official. If it appears, expect it tied to the transit network rather than open driving.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.