Is Elon Musk's Hyperloop just a pipe dream?
Much like the pun in the headline, the bright idea of transporting people using some kind of vacuum-like tube is neither new nor imaginative.
There was Robert Goddard, considered the "father of modern rocket propulsion", who claimed in 1909 that his vacuum system could suck passengers from Boston to New York at 1,200mph.
And then there were Soviet plans for an amphibious monorail - mooted in 1934 - in which two long pods would start their journey attached to a metal track before flying off the end and slipping into the water like a two-fingered Kit Kat dropped into some tea.
More recently, the Evacuated Tube Transport Technologies (ET3) system, patented in 1999, has visions of a 4,000mph trip from Beijing to New York.
They say they have attracted many investors, but as yet there is no working prototype.
So ever since inventor and entrepreneur Elon Musk hit the world's media with his plans for the Hyperloop, a healthy dose of scepticism has been in the air.
"This is by no means a new idea," says Rod Muttram, formerly of Bombardier Transportation and Railtrack.
"It has been previously suggested as a possible transatlantic transport system. The only novel feature I see is the proposal to put the tubes above existing roads."
So what chance does Mr Musk have if so many others have failed?
"I don't see anything that violates fundamental laws of physics," writes John Hansman, a professor of aeronautics and astronautics at MIT.
He says the question is not "Could you do it?" but "Could you do it in a way that makes sense from an energy-efficiency standpoint and makes sense from an economic standpoint?"
It is that economic side of Mr Musk's plans that have come under scrutiny since he announced his plans on Monday.
Indeed, unlike some of the arguably more out-there entrepreneurs of yesteryear, most are in agreement that the science of the Hyperloop, although not yet demonstrated, at least makes a good deal of sense.
Mr Musk claims his plans would come in well under the cost of the high-speed rail infrastructure being built around the world, including in the UK.
But others are far from convinced.
"From personal experience of high-vacuum systems," says Mr Muttram, "maintaining a high vacuum and the quality of alignment needed over such a long distance in an earthquake area would need some innovative engineering, and the infrastructure cost-quoted looked optimistic to me."
His concerns are shared by Pat Hanlon, a senior lecturer in transport economics at the University of Birmingham.
"Throughout history, almost all major transport innovations turn out to cost much more to develop than originally anticipated," he says.
"Only a few weeks ago we learned that the estimate for building the entire [UK high speed rail network] has increased from £30 billion to £43 billion.
"But the classic example of a cost overrun was that of Concorde, for which the total development cost turned out to be some multiple of the original estimate."
Even before investment in erecting the high-speed link could be considered, attention must be paid to research costs, says another expert.
"The technical problems involved in converting ideas like these into a commercial ground transportation system fully engineered to present-day safety standards are immense," says Alan Wickens, visiting professor at Loughborough University and a former director of research at British Rail.
"Musk's estimate of the cost of the system at £3.9bn is also extremely optimistic, even if all the technology was off-the-shelf."
Before his presentation, Mr Musk said he was not in a position to take Hyperloop forward himself, and that he was simply presenting a concept for others to pick up and develop.
In fact, such was the time burden of his current engagements - electric cars at Tesla Motors, and intergalactic ambition at Space X - that he even said he regretted ever mentioning Hyperloop.
But at the unveiling, he said he would be going ahead with making a prototype to at least show potential backers how it will work - a sign that even he perhaps realises there is a long way to go before people are fully convinced.
But progress is likely to be slow. Mr Musk has said he will get to sorting out that prototype when he gets round to it .
And even if he started today, he estimates that it would probably take about three to four years' research and development before the product would be ready, and then a further four to five years to build it.
But compared with existing plans to link Los Angeles with San Francisco, the Hyperloop comes out on top - the California High Speed Rail project is not due to be completed until 2029.
Yet while Mr Musk has the potential to win the technology battle, it would be a brave politician who would dare cancel huge investment plans in a trusted system in favour of the grand vision of one of tech's most ambitious entrepreneurs.
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