That doesn't help me, I'm afraid. It must be low enough pressure to allow lower air resistance on the train, so must be a significant vacuum. Even if not a perfect vacuum, the carriage can presumably not exchange air with it, since the carriage is at a higher pressure. If you used aircon in the train the lower pressure air in the tunnel would be very poor at convecting heat away?
Here's a great engineering project: https://en.wikipedia.org/wiki/Hawthorne_test_tunnel