Media playback is unsupported on your device Media caption Passengers would undergo forces of gravity similar to an aeroplane, Mr Musk said

US-based entrepreneur Elon Musk has unveiled his proposed near-supersonic "Hyperloop" transport concept to link Los Angeles and San Francisco.

The SpaceX, Tesla and PayPal founder envisions using magnets and fans to shoot capsules floating on a cushion of air through a long tube.

If the Hyperloop is ever built, a trip on it between the two California cities would last about 30 minutes, he said.

But Mr Musk says he is currently too busy to develop the project himself.

In a paper outlining the Hyperloop proposal, Mr Musk suggested the solar-powered system would be a faster, safer, less costly, and more efficient mode of transport between Los Angeles and San Francisco than the high-speed train currently under development.

Analysis Elon Musk has given us his vision of 21st Century intercity travel. So how would it work? He's proposing a pod in a tube solution. Ideally, you'd remove the air from the tube so the pod doesn't have to push up against it. But it's very difficult to make a vacuum tube that's 600km long. So Musk suggests just reducing the pressure and then putting a compressor on the front of the pod to pass the remaining air to the rear of the vehicle as it moves along. Some of the air would also be used to create a smooth cushion on which the pod could ride. An onboard battery would be sufficient to drive the compressor, but electric induction motors at the beginning, end and at intermediate points along the tube would be needed to accelerate, boost and decelerate the passenger cars. And all this would run on solar power. Can he make it work? He's used his aerodynamics and motor experts in SpaceX and Tesla to sketch out the initial details (PDF). What was interesting about his statement was that he is now prepared to at least build a subscale demonstrator. When Elon Musk says he'll do something, he's usually true to his word. This means we're likely to hear about Hyperloop a lot more in the future.

"Short of figuring out real teleportation, which would of course be awesome... the only option for superfast travel is to build a tube over or under the ground that contains a special environment," he wrote.

'$20 a ticket'

Mr Musk estimated the system linking the two cities, which are 380 miles (610km) apart, would cost $6bn (£3.9bn).

Capsules could depart as often as every 30 seconds and could also carry cars, travelling at up to 760mph (1,220 km/h), nearly the speed of sound.

Passengers seated in cabins inside the capsules would experience slightly more than the force of gravity, more like on an aeroplane than a roller coaster, he said on a conference call.

He said he would likely build a prototype of the concept, but not immediately, as he is busy with his commercial space project SpaceX.

A demonstration model would take up to four years to complete, he said.

He said the concept would best work between cities closer than 1,000 miles, because beyond that supersonic air travel would be preferable.

But for the shorter distance, his new concept would beat the plane, he argues, because it would not waste time ascending and descending.

The San Francisco to Los Angeles route would be elevated alongside a current California motorway, which would enable it to be constructed with minimal disputes over land rights, Mr Musk said. The tube and supporting columns would be designed to withstand earthquakes.

He estimated a trip would cost $20 (£13).