If the track is not evacuated, what happens to the supersonic shock wave off the nose of the train? Seems to me it would reflect off the ground and enclosing walls, causing all sorts of turbulence.
This is exactly what I'm trying to figure out at the moment. My guess is that the walls will be just far enough to allow the reflections to not disrupt the air immediately surrounding the vehicle, and/or there will be gaps, allowing the shocks to dissipate into the surrounding air.
I assume that a non-evacuated tunnel would require air moving at a speed that is close to (or the same as) that of the train. Keep the air circulating constantly, so it's sealed but not evacuated.
I've thought about it and I believe this is the most likely solution. The vehicle would remain aerodynamically subsonic, but could attain supersonic speeds as the air in the tunnel speeds up. This would also allow vehicles to follow one another without too much separation. Turbo fans can be used to keep the air moving throughout the loop. If enough vehicles are running at full speed, the system will always be "pressurized" as the vehicles themselves will accelerate the air in the loop.
Passenger/cargo loading would occur outside the loop (a fork) and would be brought up to speed by the power rail and inserted into the loop.
