It is literally a series of tubes

Elon Musk's wacky Hyperloop proposal

SpaceX's Elon Musk was disappointed at California's high speed rail plans, so he devised something more clever. You know those pneumatic tubes at the bank that carry your checks and money back and forth to the teller inside? Basically that, except along the length of California, with people inside. 

So crazy it just might work?

Image copyright SpaceX