Why maglev trains

One feature augurs in favor of safety: since the motor is in the track, head-on collisions are physically impossible. The motor cannot run two ways at once. The track, however, has to be built far more solidly and accurately than conventional rail is. Usefulness can also be hard to predict, because it's something that only users can decide -- and then only once a system is in place.

Critics of urban light-rail systems argue that they're practical only in very high-density environments. But this is not light rail -- even though most lines presently under construction are fairly short and are wed to such urban problems as connecting with distant air terminals. Maglev is ultimately intended for longer inter -city systems.

I once heard a friend predict that air travel would one day strangle on one limitation: airplane flight paths can spread out in the sky, but they're constrained to converge at the place where they take off or land.

Rail, on the other hand, runs between cities on one or two tracks, then spreads out where it begins or ends. The many forms of clogging that occur at air terminals are now so severe that the actual flight takes the lesser part of the time we spend on most trips.

Watch now, as rail reaches a significant fraction of airplane speeds. I usually avoid trying to make predictions -- but this technology really does have my attention.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work. In Shanghai, arguably the most famous maglev train can be found running across a stretch from Pudong International airport to the outskirts of the city. By replacing wheels and supporting machinery with electromagnets or super-conducting magnets, levitating trains are able to reach incredible speeds. Preventing interaction between wheels and rail also means less noise, vibration and mechanical failure, and fewer problems in the event of bad weather.

For all their benefits, elevated trains have largely failed to reach the mainstream in the way many back then expected. The primary challenge facing maglev trains has always been cost. While all large-scale transportation systems are expensive, maglev requires a dedicated infrastructure including substations and power supplies and cannot be integrated directly into an existing transportation system. The failed proposal for a 1,km Beijing to Shanghai maglev line in highlights this problem.

Therein lies the rub with maglev. Like ordinary magnets, these magnets repel one another when matching poles face each other. The magnets employed are superconducting, which means that when they are cooled to less than degrees Fahrenheit below zero, they can generate magnetic fields up to 10 times stronger than ordinary electromagnets, enough to suspend and propel a train.

These magnetic fields interact with simple metallic loops set into the concrete walls of the Maglev guideway. The loops are made of conductive materials, like aluminum, and when a magnetic field moves past, it creates an electric current that generates another magnetic field. Three types of loops are set into the guideway at specific intervals to do three important tasks: one creates a field that makes the train hover about 5 inches above the guideway; a second keeps the train stable horizontally.

Both loops use magnetic repulsion to keep the train car in the optimal spot; the further it gets from the center of the guideway or the closer to the bottom, the more magnetic resistance pushes it back on track. The third set of loops is a propulsion system run by alternating current power. Here, both magnetic attraction and repulsion are used to move the train car along the guideway. Imagine the box with four magnets -- one on each corner.