Transport networks really benefit from network effects; each additional line benefits from connecting to the existing network. The IBX rail project was recently proposed in New York and found to have 100k+ daily riders, which blows pretty much everything else in the US out of the water.

The other issue is that land use planning in the US except in a select few places like New York is really bad for transit. Transit can certainly be "build it and they will come", but also the land around transit needs to legally permit that as well. Rail is only really efficient at high utilization, and most US cities refuse to permit the development that makes rail useful.

This comparison picture really brings home the point that in most US cities the land use is causing rail value per dollar spent to be really low: Atlanta and Barcelona had similar population numbers and metro lengths in 1990. https://ti.org/images/AtlantavBarcelonametro.jpg