We just built a high speed train in Spain between Madrid and Barcelona which are about 400 miles apart. This is (or was) the largest air route in the world with 981 flights per week:
http://en.wikipedia.org/wiki/World%27s_busiest_passenger_air_routes
The train takes 2,5-3 hours versus 1 hour for the plane. If you take into account the fact that both train stations are located in the center of the cities and the airports are in the outskirts, it makes sense. Even more considering that the train doors close just 2 minutes before departure. The fact is that the train is taking away passengers from the plane.
I'm not a big fan of trains (even though I'm a civil engineer) but must admit that it is more confortable than the plane as you can walk, go to the bar, talk in the phone (between coaches), etc.