Hyperloop is the name of Elon Musk's newest idea. The founder of the airspace company SpaceX and electric car manufacturer Tesla wants to link San Francisco and Silicon Valley with the Central Valley and the Los Angeles metropolitan area. The train resembles a pneumatic tube, reminding some of the mail-sorting cylinders once used in office buildings.

The Hyperloop, however, is not a real pneumatic tube, but rather an environment of reduced air pressure. A total vacuum would not be possible to implement, engineers say. But regular air pumps can reduce the pressure in the tube sufficiently to enable train travel with very little air resistance and friction.

The tunnel could theoretically also go under water. This would certainly make building permits far easier to obtain. As of now, the project has hardly left the drawing board. But the company Hyperloop Technologies has started preparing a test area near Las Vegas in Nevada to buildmodels - on a smaller-than-life scale.

The slim Hyperloop vehicles are supposed to hover on air cushions, which they generate themselves by moving at speed of up to 777 miles per hour (1250 kilometers per hour). Some have doubts, however. How will passengers cope with the extreme acceleration? If travelers get sick, how would first aid reach them in those small cabins?

The idea of a partial vacuum tube is not entirely new. Swiss engineers also favor the idea for their futuristic SwissMetro project, a tube under the main metropolitan centers of the mountainous country. Those trains are also supposed to hover - not on air, but on an electromagnetic cushion. Nor would they be expected to reach Hyperloop speeds. 500 kilometers per hour should do the trick.

Select test passengers in Japan can already experience something similar to the SwissMetro plans aboard the Japanese Maglev magnetic train. This train even becomes more stable the faster it travels. The first public line between Tokyo and Nagoya is planned for 2027 - again at 500 kilometers per hour.

Maglev is short for magnetic levitation, a physical concept by which an object is suspended using a superconductor greater or equal to the gravitational force of that object. By floating instead of rolling, friction is more or less completely done away with, which makes for far less wear and tear on the vehicle.

The Shanghai Maglev Train is currently the world's fastest train in operation. It uses the same magnetic levitation technology as its Japanese counterpart and boasts an operational speed of 430 kilometers per hour. The train takes passengers from just outside central Shanghai to the airport - a journey of 19 miles - in just eight minutes.

The Shanghai Transrapid was built by Siemens and ThyssenKrupp. The original Transrapid traveled for the first time in 1983 on the test track pictured above in Northern Germany. There were several concepts for building such a line in Germany, but the groups did not get the necessary political backing. The main reason: Traditional trains with wheels on tracks were getting faster, too.

Germany instead opted for Velaro high-speed trains from Siemens. They can be easily used on existing railroad tracks. The ICE set a speed record of 405 kilometers per hour in 1988, but in practice they don't travel faster than 300. Today, Velaro trains are real export hits: They serve passengers in Spain, China, Russia, the UK and Turkey.

Velaro trains do away with locomotives. They instead use multiple motorized units located under each axle. The fastest of them travels in China: During a trial run in 2010, the CRH 380A hit a speed of 484 kilometers per hour. Commuters between Beijing and Shanghai nowadays have to settle for an operational speed of 380.

The fastest of all traditional wheeled trains remains the French TGV (Train à Grande Vitesse). It started regular operations back in 1981. The latest version - the AGV - hit a top speed of 572 kilometers per hour back in 2007. Normal TGV trains generally don't break 320. TGV-type trains can be found in Germany, Belgium, the UK, Switzerland and Italy.

Even before France, Japan launched a real high speed train: For the olympic summer games of 1964 in Tokyo, the Shinkansen started operations - then travelling at 215 kilometers per hour. Today the trains reach a maximum operational speed of 320.

Germany's Deutsche Bahn's newest Generation of ICE-4 trains are actually slower than their predecessor: 250 kilometers per hour rather than 300. They are considerably more energy efficient, though.