Bridges and overpasses are already build that way.

No you could not. The steel tube will expand and contract as temperatures change. Along most of the route, that longitudinal movement of the tube will be in the 10s and 100s of feet, which means that the "valley" could be anywhere on the tube according to temperature.

Also, tubes will resonate and vibrate as vehicles go by. This could be actively damped at the pylons though, like they already considered doing for ground subsidence.