Engines of Our Ingenuity

No. 1745:
MAGLEV TRAINS

by John H. Lienhard

Today, we ride a fast train. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Perhaps you've seen a magnetic desktop toy, built in two parts. One's an axle with two circular magnets that look like wheels, and a needle-like point on one end. The other's a holder with a small mirror on the same end. Position the point against the mirror and start the axle spinning. It floats above the holder on a magnetic field and keeps spinning for a long time. However, it's tricky, because any wobble can amplify; it's not stable.

That has much in common with the so-called Maglev train -- an idea whose time seems to've come. Maglev is short for magnetic levitation. It's a train supported by magnetic fields, which are stabilized by feedback control systems. The floating train does not touch the track, at least when it's moving at full speed.

And full speed is fast. The Japanese are building a forty-mile line that will run reach 340 miles an hour. In another episode I argue that practical land speeds are redlined at about 150 miles an hour. But this is not land transport; it's constrained flight. These trains do, quite literally, fly.

Maglev comes in different forms. It can use either magnetic attraction or magnetic repulsion to hold the train in the air. In either case, the linear motor that drives it is not part of the train, but built into the track. As with any new technology, this next generation of rail has its critics. They worry about noise, safety, and usefulness. As for noise, it appears to be less than that of conventional rail. And it's aerodynamic, not mechanical.

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.

(Theme music)


P. Sharke, Ticket to Ride. Mechanical Engineering, October 2002, pp. 46-50.

For more on Maglev trains, see, e.g.,
http://www.calpoly.edu/~cm/studpage/clottich/fund.html
http://faculty.washington.edu/jbs/itrans/maglevq.htm

For more on fast trains, see Episode 449.


magnetically levitated spindle
A magnetically levitated spindle (Photo by John Lienhard)


The Engines of Our Ingenuity is Copyright © 1988-2002 by John H. Lienhard.