First, let me say I think that estimates of cost savings over the European/Japan figures are ridiculous for anything at the ground level that moves a lot faster

First, let me say I think that estimates of cost savings over the European/Japan figures are ridiculous for anything at the ground level that moves a lot faster than state-of-the-art maglev or HSR. A closed tunnel doesn't buy you much. Mostly-evacuated tube, or at the very least some type of sealed hydraulic (which is problematic) is the obvious plausible idea to achieve supersonic speeds. The problem is structural engineering of a very large negative-pressure-sealed structure in which a fault anywhere could destroy the whole thing. It would need complex airlocks, which would make each station enormously expensive as well. I would note, however, one of my ideas that may have some synchronicity: that the speed of sound in a gas rises rapidly as the molecule gets less massive, and the drag force drops as the gas becomes less dense. Simultaneously, if hydrogen cars are going to get off the ground they need a distribution pipeline system. A train that traveled in a tube of hydrogen could effectively bypass the 'air' speed of sound and move at at least mach 3 before reaching the H2-transonic range. Sealing STP hydrogen in a container can be done an order of magnitude easier than sealing vacuum or highly pressurized gas. The "tracks", of course, would be some form of maglev. Ideally, an 'Inductrack' design, which have not been seen in use yet but offer substantial advantages. On the other hand, given the progess on high-temperature superconductor manufacturing, perhaps those advantages are shrinking. On Googling, it would appear that I'm not the only one to have this notion: http://www.supersonictubevehicle.com/docs/STV%20--%20Denizli%20%281%20Jul%2010%29.pdf http://www.supersonictubevehicle.com/docs/STV%20--%20Denizli... I don't understand attempting to use a propeller-based primer mover, however. A linear induction motor with variable coil spacing, on a multiple unit train, should function just fine for this type of use.