I guess that the air resistance against the air barrier would be even higher: if the air is moving at the speed of the train, you have the same friction per sur

I guess that the air resistance against the air barrier would be even higher: if the air is moving at the speed of the train, you have the same friction per surface unit than you would have with static air against the moving train, only your area is now much bigger. You do save the energy needed to part the air aside as the train moves forward, since the air barrier would have a constant cross-section. I don’t know how that compares to the increase in friction due to greater surface. Finally, other aspect I just realized: when you try to stop the train, you have a rush of air pushing you at several hundred Km/h, and that would be an enormous mass. You could enter a side branch which would then close behind the train and have a smaller mass of air to slow down. I’m not an engineer and have no experience on the subject, but my hunch is that it could easily end up being even more difficult.