After a few minutes playing with toy magnets, even a child quickly sees that one magnet can push or pull the other around.
But it took a rocket scientist at the turn of the past century to make the conceptual leap of using magnetic attraction and repulsion to replace the thunder of locomotives with the whoosh of trains that rarely touch track. Instead of clacking along steel rails, trains would ride on a cushion of magnetic fields at speeds of up to 300 miles an hour, propelled by manipulating the magnets' attraction and repulsion.
Now, nearly a century after Americans Robert Goddard and Emile Bachelet conceived of magnetic-levitation trains, maglev is beginning to move from the test track to the main line. In China, the United States, and Germany, plans - and, in one case, guideways - are being laid for modest systems that backers hope will demonstrate the technology's potential to unclog airports and highways.
"It's a technology whose time has come," says Christopher Brady, president of Transrapid International USA Inc. in Washington, D.C. Transrapid USA's German parent company is building a system in China, and is said to have the inside track on several projects in the US.
Research on maglev trains has been under way for decades, with Japan, Germany, and the United States spending abaout $1.5 billion each on maglev R&D. But while government-funded research ended in the US in 1975, the other two players pushed ahead.
American maglev activity received a boost, however, in the omnibus transportation bill Congress passed three years ago. Among other provisions, the bill set up ground rules for a competition that will lead to the first substantial maglev demonstration line in the US.
Last January, US Transportation Secretary Rodney Slater selected two finalists from seven competing projects.
Pittsburgh plans to run a 47-mile line connecting the city to its airport and eastern suburbs, while Baltimore plans a 40-mile stretch of guideway from Camden Yard to Union Station in Washington via the Baltimore-Washington International Airport.
The winner garners $950 million in federal grants to help pay for construction.
Last week, Transportation Secretary Norman Mineta made additional money available to Los Angeles and Las Vegas, which didn't make the final cut, to continue studies on their proposals. Atlanta, another sidelined competitor in the federal program, is pressing ahead on studies of a line to Chattanooga on its own.
Meanwhile, the US Senate's version of an economic-stimulus package contains a $7-billion bond program to fund expansion of high-speed rail, including the introduction of innovative technologies such as maglev trains.
One of the most influential states in advancing or retarding maglev trains could well be Florida. While all eyes focused on the outcome of the presidential race in Florida last year, voters overwhelmingly backed an amendment to the state Constitution that gives state officials two years to begin construction of a statewide high-speed rail system that links the state's five largest urban areas.
One of the issues planners must decide is whether they will opt for a maglev system or a conventional high-speed rail system, such as France's TGV or Amtrak's new Acela.
Currently, maglev trains fall under two basic designs, notes Don Rote, a retired physicist who worked on maglev development at the Argonne National Laboratory in Argonne, Ill.
The Germans' approach, embodied in Transrapid's design, uses magnetic attraction to levitate and propel a train. The train's wheel-free undercarriage wraps around a T-shaped guide edged with thin metal "guidance rails."
Computers and sensors control support and guidance magnets on the undercarriage that keep the train within three-eights of an inch of its guide rail on each side and about six inches above the guideway.
The underside of the guideway also contains windings that, when electrified with alternating current, generate their own set of magnetic fields, which continuously attract the support magnets.
The track's windings are switched on and off by section as the train passes, averting the need to continuously energize the entire route.The Japanese, by contrast are working on a system that uses repulsion instead of attraction.
In addition, Japanese researchers are using superconductors for magnets. Superconductors are materials that, when cooled sufficiently, offer no resistance to electric current. Thus, once a superconducting electromagnet is energized, its electrical source can be removed, and the superconductor will remain magnetized as long as it is held at the proper temperature.
As researchers discover materials that become superconductors at ever higher temperatures, the economics and simplicity of this design could improve markedly, Dr. Rote says.
Japan's design has the train riding within a U-shaped guideway. Levitation doesn't take over until the train has reached 25 miles an hour. So it uses retractable wheels at the beginning and the end of a run.
The design does allow for a wider gap between train and guideway, Rote adds, providing a more forgiving track, should it deform because of earthquake activity.
On both systems, the trains generate electricity for their "hotel" functions - heating, light, or electricity for the galley stove, for example - directly from the interaction between conductors in the train and the magnetic fields the train encounters as it moves along its guideway.
Of the two approaches, the German design is deployable now, observers say, while Japan hopes to move its version off the test track in a few years.
Maglev proponents acknowledge that the designs are best suited to runs of 40 to 600 miles - distances currently spanned by cars or short-haul airline flights.
"Since 1970, the [US] population has increased 32 percent, the number of licensed drivers has risen 64 percent, and the number of vehicle-miles traveled has grown by 141 percent," says Phyllis Wilkins, director of the Baltimore-Washington Maglev Project. "To avoid greater gridlock and winglock, we have to introduce additional means of ground transportation."
Proponents lay out a long list of potential benefits - from a bump-and-jiggle-free ride for passengers to less pollution and congestion to lower operating and maintenance costs for the rail operator, because the trains' propulsion systems have no moving parts.
"The seat covers will wear out long before the train or guide rails do," Mr. Brady quips.
But not everyone is convinced that maglev technology provides the most cost-effective means for short- and medium-haul travel.
In California, planners at the state level last month rejected maglev as an option for a high-speed network they are designing for their state.
They cited maglev's incompatibility with existing track as a major reason for opting for conventional high-speed rail approaches.
In Germany, the government recently scuttled a Hamburg-to-Berlin maglev route as too expensive, given the existence of an adequate network of conventional rail service between the two cities. Now, officials are said to be planning a more-modest route from Munich to the Munich airport.
And in Britain, Birmingham has replaced a small maglev service, which operated from 1984 to 1995 between its rail station and airport with a more conventional-shuttle train service that makes the trip at a stately 22 miles per hour. The maglev train has been donated to a museum.
Indeed, some researchers hold that if wheeled trains adopted propulsion systems similar to those used in maglev trains but without the levitation, these hybrids could reap many of the benefits of maglev without the added cost.
Looking at the alternatives, "traditionally claimed advantages of maglev were not found to hold up to wheeled-train systems incorporating similar noncontacting propulsion," according to Galen Suppes, an assistant professor in the chemical and petroleum-engineering department at the University of Kansas.
For others, however, hybrids are not radical enough.
"An incremental approach is not going to work," Ms. Wilkins argues. "We have no true high-speed rail system in the United States. Rail systems have to undergo significant changes to woo people."