I looked up a ticket on Amtrak from Miami to DC and it was almost $400. I could have hopped a cheap commuter airline for half that. Until passenger rail is cheaper than air travel, its hopes for becoming popular are doomed.
It's over a thousand miles from Miami to DC. Even if there was a real 200 MPH top speed SUPERTRAIN it would still be, at best, a 7 hour trip given intermediate stops. That's a trip some people might take for various reasons (aversion to flying, sightseeing, potential greater reliability in bad weather seasons), but it wouldn't really directly compete with air travel.