Hyperloop, Hyperpolitics: Can Elon Musk's Idea Bury CA's Rail Pork Project?
That said, the Hyperloop idea does have some problems. So let's look at the idea itself first, and then look at some pro and con arguments.
The Hyperloop approach is simply the "tube trains" of science fiction going back to Hugo Gernsback's Ralph 124c41 -- people in pneumatic tubes, traveling at high speed. The Rand Institute looked at a notion like this, using tunnels at very low air pressures, forty-odd years ago. (The Rand proposal fell down on one point: digging tunnels is expensive.)
Musk has put engineering minds to work on how such a concept could be made both technically and financially feasible. The first thing to go was tunnels (except possibly for comparatively short tunnels to cross physical obstacles). Tunneling is expensive -- I know, I said that, but tunneling is expensive, in the neighborhood of $100 million dollars per mile -- so tunnels had to go. Instead, the Hyperloop "tunnels" would be steel pipes on pylons mounted well above ground -- which simplifies another problem, because instead of needing to buy land on which to run the trains, the Hyperloop can simply follow existing highway rights of way, at least most of the way.
Inside the tunnels would be, well, nothing. Or darn close to nothing. The air pressure would be reduced to 100 Pascals -- 0.01 pounds per square inch, or about one one-thousandth of normal atmospheric pressure. Musk's note on Hyperloop notes this isn't a real vacuum, and to a space guy it's not -- but it's one tenth of the atmospheric pressure at 100,000 feet altitude, or one fifth of the atmospheric pressure on the surface of Mars. To us civilians, that's a pretty hard vacuum.
The trains themselves are single cars that would look a good bit like the body of a private jet -- streamlined aluminum tubes that fit inside the steel tunnel of the Hyperloop. At the speeds Musk envisions, up to 800 miles per hour, even the tiny air pressure inside the tubes makes aerodynamics an issue, so this becomes a design constraint: shock waves could build up between the cars and the inside of the tubes and produce a tremendous increase in drag. So the tubes must be big enough that the car is no more than about six tenths of the inner diameter of the pipes -- a seven foot diameter car needs an eleven foot tube.