Here's one of the key problems: Rail lines have to be put in, and they need land for that.
If you want to put in a a new high-speed rail line, since the current rail lines are unsuitable and prioritized for cargo, that's fine. Let's see you find a continuous strip of land going end-to-end through cities and such. They would have to expropriate countless properties, have to deal with 100s of different city planning departments, and all those similar nightmare scenarios.
Dedicated rail right of ways SHOULD HAVE been put in place when the interstate highway system was being built, but at that point in time the train was as good as dead in people's minds because Americans feverishly believed that the car was the one and only future. (Hell, they even built neighbourhoods without SIDEWALKS, because why wouldn't you just drive around the corner to the store?) Now all the urban and suburban areas are way too developed to make it happen, unless some dictatorial system does it by force.
I'd love to see it happen, but barring some miracle, it ain't gonna.