IN THE '50s it was the interstate system, in the '60s airports, and in the '70s subways. Each decade has had its own fix for America's ongoing transportation crisis. Now, with population in urban corridors booming and with the Federal Aviation Authority predicting that demand for air travel will nearly double by the end of the century, a number of states are giving serious consideration to what may be the most revolutionary transportation system since Kitty Hawk: magnetically levitating (maglev) trains.

First developed in prototype about 20 years ago, these are the trains of science-fiction legend: flying above the track on a electromagnetic cushion, noiseless, highly efficent and capable of speeds well in excess of 200 mph. In Japan and France, maglev R&D has been in high gear since the 1970s. But in the United States, development has been stalled by a lack of private and public funding.

Suddenly, however, maglevs are back, riding a new wave of interest in high-speed rail (HSR) systems. This month the state of Florida decides whether to use a maglev system developed at MIT to link Miami, Orlando and Tampa. The impetus? Transportation studies predicting, among other things, that the principal highways between Miami and West Palm Beach would have to widened to a total of 44 lanes over the next decade or so to handle surging traffic demands.

With virtually no new airports planned to handled the anticipated air travel crunch of the next 25 years (at Denver's Stapleton Airport alone the number of annual hours of passenger delays is expected to rise from 3 million to 17 million by 1996) and with the age of highway construction clearly over, no fewer than 11 states across the country are now weighing maglev technologies against the more conventional 160- to 180-mph European bullet trains like the French Train a` Grande Vitesse (TGV) to serve congested inter-city routes.

A maglev system developed in Germany is the leading candidate for the proposed HSR link between Las Vegas and Los Angeles. Michigan has held hearings on a Detroit-Chicago system. Ohio is looking at a Cleveland-Columbus-Cincinnati link. The 250-mile Houston-Dallas corridor, considered by some to be the most promising in the country, is being evaluated by a consortium of German companies.

On Capitol Hill, too, interest is spreading in the wake of a bold proposal last fall by Sen. Daniel Moynihan (D-N.Y.) to devote $300 million to maglev R&D. Moynihan, who chairs the Senate subcommittee on transportation and infrastructure, wants to see a high-speed Boston-New York-Washington link. His bill hasn't been sent to the floor yet, but already buttons are in evidence: "Maglev, Not War."

"This country used to make all the automobiles, the airplanes, and the ships. Now we hardly make any of them," says former MIT scientist Henry Kolm, the grandfather of maglev. "If we can introduce maglev in this country, we will regain a leadership position in transporation. It's that simple."

Running on Repulsion

The leading American maglev system, invented in 1970 by Kolm and fellow MIT researcher Richard Thornton, works on the simple physical principle that like magnetic poles repel each other. That same repulsion can be induced by passing a powerful magnet over a conductor, since a magnet in motion creates a mirror image of itself on appropriate surfaces. (Thus an aluminum disk which is set spinning parallel to the floor will lift or "levitate" a magnet some distance above the disk surface.) The maglev system now under consideration in Florida applies this principle to 100-passenger "magneplanes" that resemble the fuselage of a small jet. A magneplane has three superconducting magnets strapped to its underside. As it travels through an special aluminum trough, it achieves electromagnetic flight by inducing lift that keeps it suspended about a foot above the track.

Like an airplane, a maglev vehicle uses wheels only for lift-off and landing, and enjoys all the advantages of smooth ride, high speed and reduced wear (owing to absence of friction) that flight has over conventional wheeled systems.

Unlike a plane, however, the magneplane doesn't have its own engine. It is propelled by an alternating electrical current that runs through the bottom of the aluminum trough. The current is supplied by a "linear" electric motor -- that is, one which has been "unwrapped" from the normal cylindrical shape into a long horizontal strip -- which has been enhanced through the enormous strength of superconducting materials. By synchronizing the flow of current through the trough with the approaching magneplane magnets by means of trackside computers, the system creates a traveling magnetic field that runs along with the magneplane in constant reaction against the magnets, accelerating, slowing or stopping the vehicle. "The magneplane," says Kolm, "rides the magnetic wave like a surfboard."

This gives the system enormous flexibility. With power in the guideway, not the vehicle itself, individual magneplanes have very little onboard mechanical weight and don't need extensive maintenance. More importantly, they can run in any combination at any time. Instead of having one long train running once an hour between Boston and Washington and stopping at a dozen cities in between, individual magneplanes could leave once a minute out of Union Station, each making a single stop at a different city.

A second attractive factor is energy cost. Free of the friction that impedes whelled systems, but without the necessity of climbing into the atmosphere like an airplane, the magneplane requires a surprisingly small amount of current. Estimates done by the Center for Transporation Research at Argonne National Laboratory put the "energy intensity" of maglev at about one-fourth that of inter-city aircraft or automobile travel on a passenger-mile basis.

Although the magneplane was technically feasible from the moment of its invention, recent breakthroughs in superconductivity have eased the task of building and maintaining a maglev system considerably. For example, the superconducting magnets used in previous magneplane designs were kept at absolute zero through the use of heavy compressors and liquid helium. Recent advances in higher-temperature superconducting materials, however, allow the use of smaller, more efficient compressors employing liquid nitrogen instead of helium (at a cost ratio of 20 to 1) and permit a greater margin of error in maintaining temperature. Of all the potential large-scale applications for superconducting materials -- electrical transformers, generators and transmission lines -- maglev has far and away the lowest current carrying requirements. That's a major advantage, since the biggest drawback of the new high temperature superconducting materials is the relatively low amount of current they can carry before losing their superconducting properties.

"We're going to have improved materials for maglev magnets before we'll have new superconducting materials for anything else," says Larry Johnson, director of the Center for Transportation Research at Argonne National Laboratory in Illinois. "This kind of research is going to be technically feasible much sooner."

Attractive Alternatives

Meanwhile, development continues abroad. The Japanese have spent almost $1 billion over the past 20 years on a variety of maglev prototypes, with limited success. The Germans are farther ahead, and have actually been testing a maglev system for some years. Theirs, however, works on the principle of magnetic attraction, which entails certain technical problems.

The German "transrapid" system uses conventional magnets that curve around and under the crossbar of a single T-shaped monorail. When the magnets are energized, they pull themselves up towards the crossbar's metallic underside and the car is lifted into the air. Using this technique, the Germans have reached tested speeds in excess of 300 miles per hour.

One troubleseome aspect of this technology is that the cushion on which the vehicle rides -- the clearance between magnet and rail -- is only half an inch. Maintaining that clearance requires a complicated and constant adjustment of electrical current. At low speeds, that's not a problem. But at high speeds it becomes difficult and expensive. According to a study done for the Las Vegas-Los Angeles proposed system, which is the only area in North America where the transrapid system is under serious consideration, the transrapid would require about 50 percent more electricity than a high-speed system like the TGV.

More importantly, the transrapid's narrow clearance would require the same kind of constant track maintenance that has proven such a logistical and financial burden for high-speed hard-rail systems. At 160 mph, even the slightest deviation in the alignment of a track spells trouble for a bullet train. Ditto for the transrapid, which will start scraping the rail at high speeds if it isn't accurately aligned.

For transrapid's backers these aren't serious problems. "So what if it only levitate's half an inch," said Bill Dickhart, an American consultant to the transrapid company. "With a conventional wheel rail system there's zero clearance." But the fact remains that the only operational attractive system in the world is the people mover at the Birmingham airport in England, and that goes 22 mph.

Transrapid also shares conventional trains' problems in turning corners and climbing hills. All American Magneplane, the company promoting maglev in Florida, has proposed sidestepping the complicated and expensive process of obtaining new rights-of-way through heavily populated areas by building their track down the median strip of the interstate highways connecting Miami, Orlando and Tampa. Others systems can't do that. Highway curves are designed to be taken at highway speeds, not the 180-mph levels anticipated by HSR. The magneplane handles that by banking within the trough like a bobsled. In addition, it is small enough to fit under existing bridges. To follow the highway, the other two systems would have to make extensive alternations to existing overpasses.

Money Talks, Nobody Walks

All of this, however, doesn't make the magneplane a less expensive alternative to other HSR systems. Despite its apparent conceptual superiority to existing high-speed systems, the maglev has been haunted from the very beginning by the suggestion of economic impracticality. Research and development on the American maglev system has not progressed beyond the prototype built in the early '70s before OMB cut off funding for maglev work at MIT in 1975. In the interim, TGV has undergone 10 years of testing and six years of operation, and the Germans have forged ahead with their own ideas. Even magneplane proponents concede that several years of development are needed before a system could be built.

Even then, no one is convinced that the initial capital cost of a maglev system would be lower than more conventional HSR proposals. One feasibility study done in the seventies likened the cost of a maglev line in the Northeast corridor to that of a six lane highway. In Florida, the maglev proposal comes out at $3 billion, about $1 billion more than the TGV proposal. The problem is the guideway, which amounts by some estimates to about 90 percent of the total cost of the system. Not only is it constructed out of costly aluminum, but it is also elevated, which presents a fairly challenging set of engineering requirements.

Of course, maglev uses much less electricity and has much lower maintenance costs. Estimates suggest that because of its high speeds, it will easily attract more passengers and outearn slower HSR competitors.

Still, money may be hard to find. In Florida, if the maglev system wins the HSR franchise this month, the deal's backers will be given choice pieces of real estate around the proposed train stations to develop as they see fit, thus offsetting the tremendous costs of construction. But even given that, All- American Magneplane has had difficulty attracting the necessary capital.

"When the Wright brothers were around, all they needed was a barn and a bicyle factory. To do significant innovation back then, that was all you needed," says Kolm. "That's no longer possible. Now you need government funding, and the government won't fund anything that isn't demonstrably cost-effective. Well, if you consider all the great innovations in humanity, all the great ventures -- the polar expeditions, the cathedrals, the pyramids -- none of those things was demonstrably cost-effective. If nothing is done that isn't cost-effective in advance, nothing is done."