The Hyperloop has been vaguely described by Musk as a "cross between a Concorde, a rail gun, and an air hockey table." A better description might be an elevated tube system with a magnetic levitation system similar to high-speed bullet trains. The kicker would be the enclosed tube, which would provide a nearly friction-less surface for individual capsules to travel in.

ET3's Hyperloop-like project already has a number of schematics and plans already in place. They claim an automobile-sized, six-passenger capsule constructed for "outer space" travel conditions could easily reach speeds of 4,000 miles per hour on longer journeys across the country or across continents. In theory, this elevated tube system could be built for a tenth of the cost of high-speed rail and a quarter the cost of a freeway. The projected cost for a passenger to travel from Los Angeles to New York is $100.

The tubes could be connected to form a new superhighway across the United States.

Science fiction fans well remember this idea from works like Robert Heinlein's Double Star (see vacutubes and bounce tubes).

However, the basic idea is actually 200 years old this year! George Medhurst, a mechanical engineer born in 1759, had a similar idea. In Medhurst's system, however, the airtight tube contained blasts of air that propelled the carriages forward.

Here is the relevant bit from Medhurst's 1812 document Calculations and remarks tending to prove the practicability ... of a plan for the rapid conveyance of goods and passengers upon an iron road through a tube of 30 feet in area, by the power and velocity of air.

"In order to apply this principle to the purpose of conveying goods and passengers from place to place, an hollow tube or archway must be constructed the whole distance, of iron, brick, timber, or any material that will confine the air, and of such dimensions as to admit a four wheeled carriage to run through it, capable of carrying passengers, and of strength and capacity for large and heavy goods. The tube must be air tight, and of the same form and dimensions throughout, having a pair of cast iron wheel tracks securely laid all along the bottom, for the wheels of the carriage to run upon.

Update 13-Jul-2017: Take a look at a pretty good approximation to the Hyperloop idea - the the air tunnel from Through the Air Tunnel (1929) by Harl VIncent. End update.