Elon Musk, the billionaire behind PayPal and Space X, is expected to publish plans today for his supersonic hyperloop concept to connect Los Angeles and San Francisco.

The entrepreneur, who has also helped Nasa develop its Falcon rocket system, has said the hyperloop idea could cut the 380-mile journey between California’s two largest cities down to half an hour thanks to average transport speeds in excess of 700mph.

"I originally started thinking about [Hyperloop] when I read about California's high-speed rail project which was somewhat disappointing," he told a Google Hangout with Richard Branson last week.

He complained that the 130mph high-speed train connection that is currently on the table to link the two cities is unnecessarily expensive and lacking in ambition.

"It's actually worse than taking the plane. I get a little sad when things are not getting better in the future,” said Musk. "Another example would be like the Concorde being retired and the fact there is no supersonic passenger transport. I think that is sad. You want the future to be better than the past, or at least I do."

Musk announced the Hyperloop project last month and has so far revealed few details on the intercity transport system that would allow people to live in San Francisco and work in LA.

On Friday he revealed it’s a “cross between Concorde, a rail gun and an air hockey table” and “involves a tube but not a vacuum tube… not frictionless but very low friction.”

Since the project was announced speculation has been rife about how it would work but the general consensus so far is that Musk has some kind of “rail gun” in mind. This would involve passengers sat in capsules being fired down a tube, which had been stripped of most of its air. A series of magnets would accelerate and brake the cars, and also prevent them from touching the sides of the tube.

Musk has said he will not be patenting the idea and it will be “open source” so that anyone can modify it, or try to create it.

Musk also explained in the Hangout that anything over 1,000 miles would be more suited to supersonic air travel.

He said: “If I were to start another company in the future, which I don’t think is going to be anytime soon, it would be to create an aircraft that is a supersonic, vertical take off and landing electric jet.

“I think that is the ultimate form of transport. I wish someone would do that. If somebody doesn’t do that then maybe at some point in the future I will.”