The details of the most hyped transportation technology since the Segway have finally been revealed: Elon Musk's Hyperloop Alpha. It is (surprise!) a series of tubes. Specifically, low-pressure tubes suspended by pylons.
Here's how Bloomberg Businessweek explains the action inside the tubes:
[P]ods would be mounted on thin skis made out of inconel, a trusted alloy of SpaceX that can withstand high pressure and heat. Air gets pumped through little holes in the skis to make an air cushion, Musk says. The front of the pod would have a pair of air jet inlets—sort of like the Concorde. An electric turbo compressor would compress the air from the nose and route it to the skis and to the cabin. Magnets on the skis, plus an electromagnetic pulse, would give the pod its initial thrust; reboosting motors along the route would keep the pod moving.
In theory, the pods would be propelled at high speeds just below the sound barrier, and the set-up would be powered by solar panels running along the top of the system.
Musk's plans outline an initial route between Los Angeles and San Francisco with an estimated travel time of 30 minutes. His analysis suggests it could be built for under $6 billion, paying for itself in 20 years with a ticket price of $20 in today's prices.
Of course, Musk's plans are no more than that. Musk isn't planning to develop any of the technology he describes in his white paper; nor does he have any plans to tackle the difficult political and logistical problems of actually getting the system built. So while it's a nice idea, there's no way to know if it's practical without a lot more work.