One thing not mentioned here is speed. Musk has said this could go from LA to San Francisco in 30 minutes, which is an average speed of ~700 mph - slightly fas

One thing not mentioned here is speed. Musk has said this could go from LA to San Francisco in 30 minutes, which is an average speed of ~700 mph - slightly faster than a commercial airliner but in the same ballpark. I'm definitely not an expert in fluid dynamics, but it seems like safely maintaining that speed in a tube at atmospheric pressure would be difficult. Maybe there's some way to design the shape of the pods such that there's a very stable equilibrium keeping it away from the walls. Or you could potentially use a magnetic field to do this (similar to a tokamak) but that seems be trickier and more expensive than a mag-lev rail. An evacuated tube has the advantage of being much more stable at high speeds and avoids the issue of excess heat from repeatedly compressing and expanding the air in the tube. In any case, great read. I almost wonder if Musk has thrown this idea out half-baked just to get more people to start thinking outside the box about transportation...