What could have been? How could you build a train system from say, Denver to Los Angeles, Miami to New Orleans, Chicago to Dallas, that could be less than 3-4x

What could have been? How could you build a train system from say, Denver to Los Angeles, Miami to New Orleans, Chicago to Dallas, that could be less than 3-4x the flight time and have enough passengers to be remotely economical (not necessarily profitable, just not a massive drain)?