A vactrain (or vacuum tube train) is a proposed design for very-high-speed rail transportation. It is a maglev (magnetic levitation) line using evacuated (air-less) or partly evacuated tubes or tunnels. The lack of air resistance could permit vactrains to travel at very high speeds—up to 4,000–5,000 mph (6,400–8,000 km/h), which is 5–6 times the speed of sound—using relatively little power. Vactrains might use gravity to assist their acceleration. If these trains achieve the predicted speeds, the trip between Beijing and New York would take less than 2 hours, surpassing aircraft as the world's fastest mode of public transportation.
However, without major advances in tunnelling and other technology, vactrains would be prohibitively expensive.
Several articles published in China arround 2010 claim that researchers at Southwest Jiaotong University in China were developing a vactrain to reach speeds of 1,000 km/h (620 mph). However, closer examination shows that the many papers published at SWJTU never use the term vactrain, and instead refer to the ETT work of Oster, Zhang, and Wang. 
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.
- Beach Pneumatic Transit
- Atmospheric railway
- Transatlantic tunnel
- Pneumatic tube
- ET3 Global Alliance (not a 'vactrain', but a similar technology)
- Joseph Giotta (Narrator), Powderhouse Productions (April 16, 2003). "Transatlantic Tunnel". Extreme Engineering. Discovery Channel. http://dsc.discovery.com/convergence/engineering/transatlantictunnel/interactive/interactive.html.[dead link]
- Yang Xi (August 2, 2010). "Vactrain Development Underway in China". China.org.cn. Retrieved September 26, 2011.
- "Laboratory Working on Train to Run at 1,000kph". Eastday. Eastday.com. August 3, 2010. Retrieved September 26, 2011.
- Andrew Nusca (August 9, 2010). "China Developing 600 mph Airless Maglev High-Speed Train". SmartPlanet. CBS Interactive. Retrieved September 26, 2011.
- Robert M. Salter (August 1972), The Very High Speed Transit System, RAND Corporation, retrieved 2011-09-28
- Robert M. Salter (February 1978), Trans-Planetary Subway Systems: A Burgeoning Capability, RAND Corporation, retrieved 2011-09-28
- "StarTram2010". startram.com. Retrieved April 28, 2011.
- Worcester Polytechnic Institute page discussing Goddard's achievements
- Vac Trains at Orion's Arm
- Rail Journal
- Popular science
- Hydrogen Tube Vehicle
- China Plans 1,000 KPH Super Train