> Where there is enough population density and 'right of way' for trains, trains almost always make sense.
You will come really close to that if you build a road that has the same level of right of way, and drive buses. The benefit of (expensive) bridges and tunnels are not unique for trains. Some of the subway trains in Paris run on rubber tires.
Flying is better for NYC -> Colorado - LA but trains are better for NY -> DC corridor, and frankly, probably all the way down to Florida.
There is the issue of 'downtown' and 'walkable cities'. If you arrive in Baltimore near an airport, you have to rent a car. In European cities, you hop on the Subway to the Hotel, that makes a big difference as well.
But Boston->DC is ripe for train transformation.
And 'established circuits' SF->LA->Vegas should be high speed - that California can't sort that out is really sad, it's a matter of organizational/systematic incompetence.
There needs to be kind of a 'civic buy in' for it to work