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" San Francisco to Los Angeles in half an hour. Do the math and you&amp;#039;re zooming along at a speed of more than 600 miles per hour. That expedient California commute is what Tesla Motors and SpaceX chief Elon Musk is promising with his new project Hyperloop. At the moment, Musk is keeping mum on just about everything but the name and the boast, promising the big reveal of what he calls a &amp;quot;cross between a Concorde and a railgun and an air hockey table&amp;quot; on August 12.  But there are few options out there that can meet Hyperloop&amp;#039;s promised velocity. Ordinary magnetic levitation (maglev) trains like Japan&amp;#039;s speed demons can&amp;#039;t do much better than 300 to 350 mph\u2014above that speed the air resistance is simply too great. The proposed technology that seems closest to Musk&amp;#039;s promise is the vactrain, a marvel that has been promised for decades and never come close to fruition. Where Musk appears to be aiming for 600 to 700 mph, Vactrain backers like Evacuated Tube Transport seek to send passengers screaming along at speeds up to 4000 mph, fast enough to fling them cross-country in 45 minutes or from New York to Beijing in the time it takes to watch a movie. This sick speed is possible by combining frictionless maglev travel and tunnels maintained in a vacuum state so that train cars travel with essentially no resistance. Engineers have toyed with this idea for a century, and got serious about it in the 1970s, but today they are merely in an experimental, regional stage. Musk has said his system is not based upon such a vacuum. Instead, it appears that his creation might resemble a long skinny loop with air continuously circulated at ludicrous speeds\u2014perhaps like the pneumatic tube that uses compressed air to send mail zipping around a post office. (Here&amp;#039;s a 1957 PopMech story in which the company Honeywell predicted pneumatic tube travel by the year 2000.) Here&amp;#039;s the diagram, by John Gardi, that Musk has called the best guess so far as to what he&amp;#039;s up to:  Pneumatic tube transport for humans or vehicles is a staple of sci-fi, but like vactrains, it has yet to become a reality. And many of the same problems that have plagued vactrain dreamers also present major obstacles for any high-speed, long-distance train transit system, including Hyperloop. Building Whatever solution a super-high-speed train uses to overcome air resistance, the first major issue is just having enough power to keep it going, says John Mansman, MIT engineering professor and transportation expert. In the case of a vactrain, he says, &amp;quot;just the electrical cost of keeping [the vacuum] evacuated in not trivial&amp;quot;\u2014constantly pumping air would probably require more energy than operating the maglev system itself. What about Musk&amp;#039;s train? If speculation is correct that Hyperloop will keep air cycling through the system at hundreds of miles per hour, then the mechanics of keeping that air blowing won&amp;#039;t be cheap, either. And don&amp;#039;t forget the maintenance costs of either A) a vactrain having to keep hundreds of miles of vacuum tunnel leak-proof, or B) maintaining the &amp;quot;air hockey&amp;quot; part of Musk&amp;#039;s system\u2014the column of air needed to propel the train through the straightaway. Rights A tunnel between L.A. and San Francisco will pass under a lot of private and public land. And at 600 mph, the train can&amp;#039;t zigzag around places where owners didn&amp;#039;t agree to sell their rights. Those cars have got to go in a straight line. (The Verge reports that Hyperloop would have a turning radius of 40 miles.) Jim LaRusch, attorney for the American Public Transportation Association, says this is an illustration of why huge transportation projects are typically government-funded, or at least have the government as a partner: It&amp;#039;s much, much easier to get the rights in order. For example, in parts of California, LaRusch says, the government can reimburse utilities if a utility must move its underground infrastructure to make way for a tunnel. A private firm is on its own. &amp;quot;There&amp;#039;s got to be somebody with a sewer, or a cable line, or something in the way,&amp;quot; he says. In Musk&amp;#039;s case, he&amp;#039;s fortunate to be building Hyperloop within one state and avoiding the legal tangle of trying to secure rights across states, as a nation-spanning vactrain would have to do\u2014states and cities have a hodgepodge of different laws covering these property rights.  Still, LaRusch says, the rights question is an open one, especially if Musk isn&amp;#039;t working with the State of California. &amp;quot;It&amp;#039;s not really been tested,&amp;quot; he says. &amp;quot;Not a lot of people have tried it without a government piece.&amp;quot; Even the transcontinental railroad of the 1800s, perhaps the closest analog to a nation-spanning underground rail, was possible only because of using government land grants to the rail companies, LaRusch says.  From the December 1957 PopMech. Safety What happens when a train that uses electric power to brake is traveling hundreds or thousands of miles per hour and the power goes out? &amp;quot;The thing is then scraping along the bottom of the track at [incredible speeds],&amp;quot; Mansman says. That&amp;#039;s why backup power, and using any means necessary to avoid a blackout, is so important for a super-high-speed train. &amp;quot;The trains have to be space vehicles,&amp;quot; Mansman says of vactrain systems, because sending humans flying through a vacuum tunnel in a pressurized can at thousands of miles per hour presents a few key safety challenges. First, the train has to be able to withstand extreme deceleration in the case of an emergency stop\u2014riders must be able to survive a bump off the wall and a hard slowdown. Second, the train car can&amp;#039;t suffer any leaks even in a crash, since it&amp;#039;s sitting in a vacuum tunnel. Third, Mansman says, the car needs to have a sufficient backup oxygen system in case it takes a long time for crews to rescue the vactrain passengers. &amp;quot;You&amp;#039;re stopped 500 miles from the nearest station, with limited oxygen,&amp;quot; he says, and letting air back into such a huge system could take hours. Here Musk would have a leg up. Hyperloop would avoid the dangers of air supply if it&amp;#039;s not a vacuum system. Musk still must account for the risk of an accident at 600-plus mph, however statistically low it might be, but the &amp;quot;loop&amp;quot; part of Hyperloop&amp;#039;s design would mean there&amp;#039;s no hard terminus to worry about. Money Simply, enormous underground rail costs a fortune. &amp;quot;There&amp;#039;s not a basic physics issue,&amp;quot; Mansman says. Super-high-speed trains can be built\u2014it&amp;#039;s a matter of making the business case for them.  Just building the Chunnel 30 miles between England and France cost several billion dollars, he points out. The trip from San Francisco to Los Angeles is more than 10 times as far, never mind crossing the country or the ocean as an even more ambitious system might. &amp;quot;It&amp;#039;s how you build up infrastructure over the distances that are important,&amp;quot; he says.  We&amp;#039;ll see what Musk says about the dollars and cents in August. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/technology/infrastructure/a9225/hyperloop-and-friends-why-dont-we-have-super-high-speed-rail-already-15710308/>