HALF a century after they were pioneered in France and Japan, could high-speed trains be coming to America? Last year California’s legislators gave $7.7 billion to a project called California High Speed Rail (CHSR). If and when it is completed it will connect San Francisco to Los Angeles, with branch lines to Sacramento and San Diego. This first slice of what the budget suggests will eventually be a bill for $68 billion will be used to construct 210km (130 miles) of track between Fresno and Bakersfield. But just because the money has been allocated does not mean the line will actually get built. It is far from universally popular. Besides the estimated price (and even this is probably a shot in the dark, for big infrastructure projects are hardly known for coming in on budget), it may not even be that fast. The short distances between many of its stations mean trains will rarely be able to reach their planned top speed of 350kph.

Fortunately, California is home to many clever people. One of them is Elon Musk, the hyperactive boss of Tesla Motors, an electric-car company, and SpaceX, a rocketry business (he also sits on the board of Solar City, a solar-energy firm). There is nothing Mr Musk likes more than revolutionising high-tech industries. And he thinks he has come up with a better way to get California moving than a standard high-speed train. Mr Musk—who is as good at PR as he is at engineering—first mentioned his idea, called Hyperloop, last year, prompting excited speculation about what he might have had in mind.

Mr Musk’s atmospheric caper

On August 12th, in a short document published on the websites of Tesla and SpaceX, all was revealed. Essentially, Mr Musk proposes to revive an old science-fiction idea called the vac-train (short for “vacuum train”), albeit with a few important tweaks. The Hyperloop would carry passengers across California at more than 1,200kph—faster than a jet airliner—allowing them to zoom between San Francisco and Los Angeles in little over half an hour, compared with more than two-and-a-half hours for CHSR. It would be solar-powered, would take less land than a high-speed railway, and would be cheaper to boot. Mr Musk’s notional budget is around $6 billion, less than a tenth of what the high-speed train is supposed to cost.

Vac-trains, as first described in the 1910s by Robert Goddard (better known as a pioneer of rocketry), would send rolling stock (or hovering stock, perhaps) hurtling through hermetically sealed tubes from which the air has been evacuated. The trains would thus encounter no drag, and be able to reach immense speeds. Goddard reckoned his design—which also proposed magnetic levitation instead of wheels—was good for about 1,600kph.

This and other designs for transporter tubes inspired much futuristic art, as the illustration above suggests. But none was ever built because maintaining a vacuum in a long tunnel is difficult. Pumps must work exponentially harder as the pressure falls, to evacuate the few air molecules that remain. And even a small leak would scupper a full-fledged vac-train, which relies on no air at all being able to build up in front of it, and thus slow it down. For that reason, the Hyperloop is not actually a true vac-train. Instead, Mr Musk plans to remove sufficient air from the tubes to give them a pressure roughly a sixth of that on the surface of his beloved Mars, or a thousandth of that on Earth at sea level. This would keep the air resistance low enough to deal with in other ways.

The chief of these would be to suck up the air that did accumulate in front of the tube’s rolling stock (putatively, individual pods that could hold 28 people each) using a fan, and then expel most of it from the pod’s rear end. Some of this air, though, would be diverted out of the sides through special skis, to create a cushion that would stop the pod touching the tunnel walls.

Each pod would be launched by a linear-induction motor (such motors are also being tested for use as catapults on aircraft carriers), and booster motors every 110km would keep its speed up. On reaching its destination, the pod would pass through a motor that worked in reverse, converting its kinetic energy back into electricity for storage in batteries or use in motors up the line. And, this being California, the whole thing would naturally be powered by solar panels mounted on the roof of the tube.

Loop the loop

That, at least, is the theory. There are doubters, of course. Some worry that passengers will not like the prospect of hurtling through a steel tube, in a cramped capsule, at almost the speed of sound. And there are inevitable questions about safety, though the pods would have wheels that could be deployed if needed, allowing them to limp to their destinations using batteries if the power failed. But, its breathtaking audacity aside, the thing does look feasible as an engineering project.

The tube would be held above ground, on pylons, reducing the amount of land it consumed, and would follow existing roads, which should simplify construction and make maintenance easier. The proposed route features only gentle curves. And the air cushion surrounding each pod should ensure that the ride is smooth. Moreover, although unexpected engineering problems would be bound to crop up, Mr Musk’s experience—and that of his engineers—with space flight and car design would bode well for overcoming them.

With projects like this, though, good engineering is never enough. Politics and economics are more forbidding obstacles. Being new, the Hyperloop is risky. Also, the CHSR has a tortuous history going back decades. Much political and reputational capital is invested in it. To replace it now with a completely different design would require an agility that California’s government is almost certainly incapable of. Nor is there any reason to believe that Hyperloop would be immune to the hypertrophication of cost that every other grand infrastructure project seems doomed to suffer. Building it alongside existing roads would certainly cheapen things as well as simplifying them, but critics who are poring over Mr Musk’s cost estimates, for everything from land permits to the construction itself, doubt the numbers stack up (though to be fair, both his electric cars and his space rockets have come in on budget). A few, presumably not Californian patriots, have even suggested that somewhere like Texas—where the bureaucracy is less stifling—might be a more feasible place to try the idea out.

Lastly, it is not clear just how serious Mr Musk really is. In the past he has said that, given his other commitments, he lacked the time to try to build the Hyperloop himself. The reason for putting it into the public domain was therefore to give someone else the chance to take it up. But it is hard to think of anyone else who has both his deep pockets and his technical track record. Or at least, that was the case before August 12th. In a conference call that day discussing the idea, he admitted that his thinking may be changing, at least a little. “I think it might help if I built a demonstration article,” he mused. “So I think I probably will do that, actually.”