The Metromover is Hitachi's driverless people mover, the kind of overhead automated transit you find running loops through a real city downtown. It appeared in the trailer as part of Leonida's urban backdrop, which is a nice touch of realism. This is a service vehicle in the truest sense, so it is almost certainly part of the world rather than anything you climb into and drive. Consider it scenery with a schedule.
The Metromover was confirmed by trailer footage as part of the city environment. Automated transit like this is built to run its own route, not to be commandeered, so there is no known way for a player to control one. We would file this firmly under world detail rather than usable vehicle.
If Rockstar ever lets you interact with it, we will note that here. For now, enjoy it as part of the skyline.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.