A vactrain (short for vacuum tube train) is a proposed type of high-speed transport system where vehicles travel through partially evacuated tubes or tunnels. By operating in a partial vacuum, air resistance is drastically reduced, allowing for higher speeds than conventional ground transportation, theoretically up to 4,000 km/h (2,500 mph). Vactrains have been proposed from 1906, and developed as scale models since 1917. Most vactrain concepts use maglev technology to lift and propel the vehicle.
18th century
In 1799, George Medhurst of London conceived of and patented an atmospheric railway that could convey people or cargo through pressurized or evacuated tubes. The early atmospheric railways and pneumatic tube transport systems (such as the Dalkey Atmospheric Railway) relied on steam power for propulsion.
19th century In 1888, Michel Verne, son of Jules Verne, imagined a submarine pneumatic tube transport system that could propel a passenger capsule at speeds up to 1,800 km/h (1,100 mph) under the Atlantic Ocean (a transatlantic tunnel) in a short story called "An Express of the Future".
20th century The vactrain proper was invented by Robert H. Goddard as a freshman at Worcester Polytechnic Institute in the United States in 1904. Goddard subsequently refined the idea in a 1906 short story called "The High-Speed Bet", which was summarized and published in a Scientific American editorial in 1909 called "The Limit of Rapid Transit". Esther, his wife, was granted a US patent for the vactrain in 1950, five years after his death. In 1909, Russian professor Boris Weinberg 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 1,666 km/h (1,035 mph). 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 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 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 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 4,800 km/h (3,000 mph) and subject passengers to accelerations 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.
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