Never let it be said that Elon Musk, a serial entrepreneur who was a co-founder of PayPal and the electric car company Tesla Motors, is afraid to think big.
Mr. Musk on Monday harked back to the days of the late-1990s tech bubble — when no idea seemed too big or too expensive — by showing off plans for a project that seems the stuff of science fiction.
The hypothetical project is called the Hyperloop, a high-speed train that would take people to San Francisco from Los Angeles in 30 minutes. That is a speed of almost 800 miles an hour.
The first unanswered question among many for Mr. Musk’s ambitious vision is who exactly would build this 400-mile transportation system. Mr. Musk suggested someone else should do it. But if no one takes the baton, he might do it. Or not.
Beyond that, the details of who would pay for Hyperloop, how it would be built and how long it would take are also unclear. But Mr. Musk theorized that if he devoted all of his energy, he could have a prototype done within one to two years. He estimated the project would cost around $6 billion and tickets would cost $20 per trip.
“It doesn’t seem plausible to me,” said Richard White, a professor of American history at Stanford and author of “Railroaded: The Transcontinentals and the Making of Modern America.” “I’m suspicious about everything, especially cost.”
Mr. White added, “How’s he going to build this thing for $6 billion? You can’t even build the Bay Bridge for that much money.” The still-unfinished renovations of the Bay Bridge connecting San Francisco and Oakland are expected to cost $6.3 billion.
So don’t pack your bags just yet. In an interview with Bloomberg Businessweek on Monday, Mr. Musk said he regretted mentioning the Hyperloop last year, saying that he has no time to work on the project and instead has to run SpaceX and Tesla Motors, his two other companies.
Mr. Musk first mentioned Hyperloop last summer and detailed it further in an interview with Bloomberg Businessweek. In July, he announced on Twitter that he would unveil the designs for the high-speed train on Aug. 12. As promised, a 57-page “alpha design” plan was posted online Monday that explained how such a train would work.
Mr. Musk has clearly put a lot of thought into the design. The document he unveiled explains that the high-speed train would become “truly a new mode of transport — a fifth mode after planes, trains, cars and boats.” The Hyperloop would transport people in “pods” that would travel through tubes. The tubes would be mounted on pylons that could be designed to withstand earthquake movements.
Mr. Musk took swipes at the California High Speed Rail that is being built and headed by the California High-Speed Rail Authority. This train, while real, is not expected to be completed until 2029 and will cost an estimated $68.4 billion to build.
“When the California ‘high speed’ rail was approved, I was quite disappointed, as I know many others were too,” Mr. Musk wrote, while saying that the Hyperloop would cost $6 billion to build. It is not clear how he arrived at this cost estimate.
If anyone could build such a train, it is probably Mr. Musk. Critics railed against him when he first broached the idea for private space travel with Space Exploration Technologies, of Hawthorne, Calif. Otherwise known as SpaceX, Mr. Musk’s company proved critics wrong last year when it launched its Falcon 9 rocket.
But Mr. Musk’s assertion that he does not want to be the leader of the Hyperloop project has some people wondering if it will actually be built.
During a news conference, Mr. Musk seemed to waver over whether he wanted to be involved with the project. “I’m somewhat tempted to at least make a demonstration prototype,” he said. “I’ve sort of come around a little bit on my thinking here that maybe I should do the beginning bit and build a subscale version that’s operating.”
In the paper released Monday, Mr. Musk acknowledged that there had been other proposed ideas for a train similar to the high-speed train over the years. “Unfortunately,” he wrote, “none of these have panned out.”