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A vactrain (or vacuum tube train) is a proposed design for very-high-speed rail transportation. It is a maglev (magnetic levitation) line using partly vacuum tubes or tunnels. Reduced air resistance could permit vactrains to travel at very high (hypersonic) speeds with relatively little power—up to . This is 5–6 times the speed of sound in Earth's atmosphere at sea level.
In 1909, Russian professor built the world's first model of his proposed version of the vactrain at Tomsk Polytechnic University. He later published a vactrain concept in 1914 in the book Motion without friction (airless electric way).
In 1955, in his novel The Magellan Nebula, Polish science-fiction writer Stanisław Lem wrote about an intercontinental vactrain called "organowiec", which moved in a transparent tube at a speed higher than . In April 1962, the vactrain appears in the story "Mercenary" by Mack Reynolds, where he mentions Vacuum Tube Transport in passing.
During the 1970s, a leading vactrain advocate, Robert M. Salter of RAND Corporation, published a series of elaborate engineering articles.
An interview with Robert Salter appeared in the Los Angeles Times (June 11, 1972). He discussed, in detail, the relative ease with which the U.S. government could build a tube shuttle system using technologies available at that time. Maglev being poorly developed at the time, he proposed steel wheels. The chamber's door to the tube would be opened, and enough air admitted behind to accelerate the train into the tube. Gravity would further accelerate the departing train down to cruise level. Rising from cruise level, the arriving train would decelerate by compressing the rarefied air ahead of it, which would be vented. Pumps at the stations would make up for losses due to friction or air escaping around the edges of the train, the train itself requiring no motor. This combination of modified (shallow) gravity train and atmospheric railway propulsion would consume little energy but limit the system to subsonic speeds, hence initial routes of tens or hundreds of miles or kilometers rather than transcontinental distances were proposed.
Trains were to require no Railway coupling, each car being directly welded, bolted, or otherwise firmly connected to the next, the route calling for no more bending than the flexibility of steel could easily handle. At the end of the line, the train would be moved sideways into the end chamber of the return tube. The railway would have both an inner evacuated tube and an outer tunnel. At cruise depth, the space between would have enough water to float the vacuum tube, softening the ride.
A route through the Northeast Megalopolis was laid out, with nine stations, one each in Washington DC, Maryland, Delaware, Pennsylvania, New York, Rhode Island, Massachusetts, and two in Connecticut. Commuter rail systems were mapped for the San Francisco and New York areas, the commuter version having longer, heavier trains, to be propelled less by air and more by gravity than the intercity version. The New York system was to have three lines, terminating in Babylon, Paterson, Huntington, Elizabeth, White Plains, and St. George.
Salter pointed out how such a system would help reduce the environmental damage being done to the atmosphere by aviation and surface transportation. He called underground Very High Speed Transportation (tube shuttles) his nation's "logical next step". The plans were never taken to the next stage.
At the time these reports were published, national prestige was an issue as Japan had been operating its showcase shinkansen for several years and maglev train research was hot technology. The American Planetran would establish a transcontinental subway service in the United States and provide a commute from Los Angeles to New York City in one hour. The tunnel would be buried to a depth of several hundred feet in solid rock formations. Construction would make use of to ensure alignment and use tungsten probes to melt through igneous rock formations. The tunnel would maintain a partial vacuum to minimize drag. A trip would average and subject passengers to accelerations up to 1.4 times that of gravity, requiring the use of compartments. Enormous construction costs (estimated as high as US$1 trillion) were the primary reason why Salter's proposal was never built.
Starting in the late 1970s and early 1980s, the Swissmetro was proposed to leverage the invention of the experimental German Transrapid maglev train, and operate in large tunnels reduced to the pressure altitude of at which the Concorde SST was certified to fly.
In the 1980s, Frank P. Davidson, a founder and chairman of the Channel Tunnel project, and Japanese engineer tackled the transoceanic problems with a proposal to float a tube above the ocean floor, anchored with cables (a submerged floating tunnel). The transit tube would remain at least below the ocean surface to avoid water turbulence.
On November 18, 1991, Gerard K. O'Neill filed a patent application for a vactrain system. He called the company he wanted to form VSE International, for velocity, silence, and efficiency. However, the concept itself he called Magnetic Flight. The vehicles, instead of running on a pair of tracks, would be elevated using electromagnetic force by a single track within a tube (permanent magnets in the track, with variable magnets on the vehicle), and propelled by electromagnetic forces through tunnels. He estimated the trains could reach speeds of up to – about five times faster than a jet airliner – if the air was evacuated from the tunnels. To obtain such speeds, the vehicle would accelerate for the first half of the trip, and then decelerate for the second half of the trip. The acceleration was planned to be a maximum of about one-half of the force of gravity. O'Neill planned to build a network of stations connected by these tunnels, but he died two years before his first patent on it was granted.
ET3 claim to have achieved some work that resulted in a patent on "evacuated tube transport technology" which was granted in 2009. They presented their idea in 2013 on public stage.
In August 2013 Elon Musk, CEO of Tesla and SpaceX, published the Hyperloop Alpha paper, proposing and examining a route running from the Los Angeles region to the San Francisco Bay Area, roughly following the Interstate 5 corridor. The Hyperloop concept has been explicitly "open-sourced" by Musk and SpaceX, and others have been encouraged to take the ideas and further develop them.
To that end, a few companies have been formed, and several interdisciplinary student-led teams are working to advance the technology. SpaceX built an approximately scale model track for its pod design competition at its headquarters in Hawthorne, California.
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