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A vactrain (or vacuum tube train) is a proposed, as-yet-unbuilt design for future high-speed railroad transportation. It is a maglev line run through evacuated (air-less) or partly evacuated tubes or tunnels. The lack of air resistance could permit vactrains to use little power and to move at extremely high speeds, up to 4000–5000 mph (6400–8000 km/h, 2 km/s), or 5–6 times the speed of sound (Mach 1) at standard conditions. Though the technology is currently being investigated for development of regional networks, advocates have suggested establishing vactrains for transcontinental routes to form a global network.
Vactrain tunnels could permit very rapid intercontinental travel. Vactrains could use gravity to assist their acceleration. If such trains went as fast as predicted, the trip between Beijing and New York would take less than 2 hours, supplanting aircraft as the world's fastest mode of public transportation.
Travel through evacuated tubes allows supersonic speed without the penalty of sonic boom found with supersonic aircraft. The trains could operate faster than Mach 1 without noise.
However, without major advances in tunnelling and other technology, vactrains would be prohibitively expensive. Alternatives such as elevated concrete tubes with partial vacuums have been proposed to reduce costs.
Researchers at Southwest Jiaotong University in China are developing (as of 2010) a vactrain to reach speeds of 1,000 km/h (620 mph). They say the technology can be put into operation in 10 years.
The modern concept of a vactrain, with evacuated tubes and maglev technology, was explored in the 1910s by American engineer Robert Goddard, who designed detailed prototypes with a university student. His train would have traveled from Boston to New York in 12 minutes, averaging 1,000 mph (1,600 km/h). The train designs were found only after Goddard's death in 1945 and his wife filed for the patents.
Russian professor Boris Weinberg offered a vactrain concept in 1914 in the book Motion without friction (airless electric ways). He also built model of his proposed transport in Tomsk university in 1909.
An interview with Robert Salter appeared in the LA 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 couplers, 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 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 bullet train for several years and maglev train research was hot technology. The American Planetran would establish 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 lasers 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 3,000 mph (4,800 km/h) and subject passengers to forces up to 1.4 times that of gravity, requiring the use of gimballed 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 '80s, the Swissmetro was proposed to leverage the invention of the experimental German Transrapid maglev train, and operate in large underground tunnels reduced to the pressure altitude of 68,000 feet (21,000 m) 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 Yoshihiro Kyotani (ja) tackled the transoceanic problems with a proposal to float a tube above the ocean floor, anchored with cables. The transit tube would remain at least 1,000 feet (300 m) below the ocean surface to avoid water turbulence.
Dr. James Powell, former co-inventor of superconducting maglev in the 1960s, has since 2001 led investigation of a concept for using a maglev vactrain for space launch (theoretically two orders of magnitude less marginal cost than present rockets), where the StarTram proposal would have vehicles reach up to 8,900 mph (14,300 km/h) to 19,600 mph (31,500 km/h) within an acceleration tunnel (lengthy to limit g-forces), considering boring through the ice sheet in Antarctica for lower anticipated expense than in rock.
ET3 (Evacuated Tube Transport Technologies) propose a system in which ″Car sized passenger capsules travel in 1.5m (5') diameter tubes on frictionless maglev. Air is permanently removed from the two-way tubes that are built along a travel route. Airlocks at stations allow transfer of capsules without admitting air. Linear electric motors accelerate the capsules, which then coast through the vacuum for the remainder of the trip using no additional power. Most of the energy is regenerated as the capsules slow down. ET3 can provide 50 times more transportation per kWh than electric cars or trains″.
Vactrains have occasionally appeared in science fiction novels, including the works of Arthur C. Clarke (Rescue Party, 1946), Ray Bradbury (Fahrenheit 451, 1950), Peter F. Hamilton (The Night's Dawn Trilogy), Joe Haldeman (in his novel Buying Time), Larry Niven (A World Out of Time), Robert A. Heinlein (Friday), Jerry Yulsman (Elleander Morning), and Jasper Fforde (the Thursday Next novels). Flash Gordon (1947) and the movie Logan's Run (1976) featured similar high-speed transport trains. The Space: 1999 TV series, featured a Lunar Vactrain. 23rd century San Francisco has one stretching across the Golden Gate Bridge in Star Trek: The Motion Picture (1979). Earlier Gene Roddenberry television productions, Genesis II and Planet Earth, featured such transport systems. In Starship Troopers (1997), actress Denise Richards uses what could be a vactrain system at 'Federal Transport Hub 39', after signing up as a pilot, and also in an earlier deleted scene.
A fictional train that matched a vactrain in description was mentioned in the 1982 song "I.G.Y. (What a Beautiful World)", by the American singer and songwriter Donald Fagen. The song includes the lyrics: "On that train, all graphite and glitter / Undersea by rail / Ninety minutes from New York to Paris".