Trains That Can Levitate

One item is on everyone's list of potential benefits of high-temperature super-conductors: maglevs, or magnetically levitated superfast trains. It is a safe prediction, since the new materials give promise of electromagnets far more powerful and economical than those in use today. And it is the electromagnet that lifts and propels existing maglevs in Japan, West Germany and Britain.

As long ago as 1979 an unmanned Japan Railways Group prototype fitted with low-temperature superconducting electromagnets hit 321 m.p.h. on a test track; a version carrying three passengers made it to 249 m.p.h. earlier this year. That beats any conventional rival, including Japan's celebrated bullet train, which goes as fast as 149 m.p.h., and the French TGV, which provides the world's fastest regularly scheduled rail service, at speeds of up to 186 m.p.h.

Japan's maglev is faster because instead of pounding along a set of rails, it floats four inches above a guideway on a cushion of magnetic force; there is no friction to slow it down, no fear of derailment on a section of bent track. This maglev has wheels, but the only times it uses them are while picking up speed before lift-off and while slowing down after landing.

The principle behind the maglev is simple: opposite magnetic poles attract each other; like poles repel. In Japan's version, eight superconducting electromagnets are built into the sides of each train car, and thousands of metal coils are set into the floor of the guideway. When the train is in motion, the electromagnets on the train induce electric currents in the guideway coils, which then themselves become electromagnets. As power is increased, the opposing sets of magnets repel each other and lift the train into the air. Two other rows of electromagnets, one on each wall of the U- shaped guideway, repeatedly reverse polarity to push or pull on the coach's magnets and thus move the train forward.

In planning the train, Japanese engineers chose superconducting magnets ( because for a given input of electricity they generate more intense magnetic fields -- and thus greater lifting and propulsion power -- than conventional electromagnets. The drawback: the liquid-helium coolant needed for the superconducting magnets is expensive, and a heavy compressor is required in each coach to reliquefy the evaporating helium. That is why maglev engineers are excited by the idea of the new high-temperature superconductors, which would use considerably less expensive liquid nitrogen as a coolant and require far smaller compressors. The developments of the past few months, says Research Chief Kazuo Sawada, who has been in on the project from the beginning, are a "promising sign."

In West Germany, on the other hand, the new superconductors are of little interest to maglev engineers, who abandoned superconducting magnets in 1979. They opted to use conventional electromagnets instead. The German system is based on magnetic attraction, not repulsion. The magnets are on assemblies attached to the cars' undercarriages that curve around and under the crossbar of a T-shaped track. When the magnets are energized, they pull themselves up toward the crossbar's metallic underside and the car is lifted into the air; magnets in the track provide propulsion. Which technique is better? Both have advantages. The German maglev is simpler and less expensive to operate. But so far the Japanese trains are about 100 m.p.h. faster.