By Li Hongmei

US billionaire Elon Musk unveiled plans on Monday for a futuristic "Hyperloop" transportation system to whisk passengers between San Francisco and Los Angeles in under half an hour.

The highly anticipated announcement put to rest some of the questions surrounding the ambitious project, which the Tesla Motors founder and CEO has hinted at for months but declined to discuss in detail.

Musk said the Hyperloop would cost less than US$6 billion in total and could transport 7.4 million people each way each year.

He said it would take seven to 10 years for the project to be completed.

The solar-powered system, which Musk previously described as a cross between a Concorde, rail gun and air-hockey table, would shoot as many as 28 passengers in each enclosed capsule through a low-pressure steel tube at up to 1,200km/h, according to the 57-page design plan.

Musk, who made his name as a PayPal founding member before going on to start SpaceX and Tesla, envisions capsules departing every 30 seconds and traversing the roughly 640 kilometres between Los Angeles and San Francisco - the busiest traffic corridor on the West Coast - along an elevated tube erected along the I-5 interstate highway.

The capsules ride a cushion of air blasted from "skis" beneath, propelled via a magnetic linear accelerator.

Major questions remain, notably whether the California state government will ever approve the massive project, and whether any private companies are willing to step in and build it. The design remains theoretical and has yet to be tested in the field.

Musk has said he is too busy running electric car company Tesla and rocket manufacturer SpaceX to build the Hyperloop himself. He said the design plans were open-source, meaning others can build on them.

On Monday, however, he told reporters on a conference call he could kick off the project.

The Hyperloop would be safer, faster, less expensive and more convenient, Musk said in a blog post.

The expected half-hour travel time for Hyperloop passengers compares with current travel times of an hour and 15 minutes by plane, about five and a half hours by car, as well as about two hours and 40 minutes via California's planned high-speed rail.