The dirty little secret is we've basically done that already - building train lines or subways in the US is so astronomically expensive that no one is doing it "for profit" anymore, and it looks likely that it'll never become financially viable unless something changes massively. I mean, from what I can tell NYC can't profitably retrofit the subways, forget about building a new line. Amtrack is constantly in bankruptcy or being bailed out. No one is going to build a modern train line from Rochester NY to NYC again - there just isn't going to be the passengers.