“You would have giant pumps keeping a near vacuum in the tube, probably 20 to 30 miles apart (30-40km)” explains Prof Frankel. “The main areas for leaks would be the end stations. The train would pass through a seal as it enters and leaves the tube.”
In fact, the train would have to pass through a series of airlocks that progressively reduce the pressure until the train enters the fully evacuated tunnel, where it could accelerate to top speed.
Ultimately, the huge cost of building such a system was its downfall along with the fact that the top speed was equivalent to existing bullet trains in countries like China and Japan.
But combing vacuum train with another technology overcomes the speed barrier. A few weeks ago, I wrote about maglev (magnetic levitation) trains, vehicles that float on a bed of magnets to cut down friction. In standard set-ups these have achieved speeds of up to 581km/h (361mph). But in a vacuum, some believe they could reach speeds of more than 4,000km/h.
Again, several systems that combine these two technologies have been proposed over the years. But today, one of the main proponents is American engineer Daryl Oster, who holds the patent for what he calls Evacuated Tube Transport. His design is a six-person capsule, roughly the size of a car, which is shot along a 1.5m diameter vacuum tube using maglev technology.
He has proposed various configurations and designs from low tech, 200mph systems for local to high tech 4,000mph systems for continental and intercontinental transport. That would mean a trip from the US to China would take just a couple of hours, revolutionizing both passenger and freight transport.
He also says that it would not cost as much as people think. He quotes a 2003 study which calculated the cost of a 350mph system to be about $2million per mile, around the same basic cost of high-speed rail projects proposed in the US, although this does not take into account maintenance, personnel or rolling stock.
Oster’s company does not propose building tracks itself. Instead, it sells licenses to the patented technology. So far, it says, it has sold more than 60 licenses, 12 of which are in China which is also pursuing vac-train research.
It may all sound rather far-fetched, but Frankel says that the time is right for vaccum-trains.
“Our rail technology is almost 100 years old,” he says. “Our airways are becoming terribly congested, and getting to, from and through airports is very time consuming.
“A train that goes from city center to city center could be advantageous.”
Daryl Oster agrees. He believes we could be using ETT for world travel in less than 10 years, with the most attractive routes between major cities. Ideally these would be separated by dry, flat, unpopulated terrain, where the ground doesn’t freeze. He believes India and China are currently the most promising looking places for what he modestly calls "space travel on Earth".
But that moniker may hold true if Dr James Powell, the co-inventor of Maglev technology, gets his way. He has proposed a system called Startram that uses a maglev vacuum train to launch objects into space. The project would require a huge cannon-like tunnel to accelerate a launch vehicle to speeds of at least double those of the ETT to get into orbit. If it is built, Powell says, it could slash the cost of putting vehicles into space. Now, that would be real vacuum travel.