MIYAZAKI, Japan (AP) _ After years of research and tests, the Japanese are preparing to build a commercial version of a supertrain that flies inches above a trackway on superconducting magnets at more than 300 miles an hour.

It takes just 30 seconds for it to reach a speed of 180 mph.

Getting the train into service would put the Japanese in the forefront of development of superconductive materials, which carry electricity without any resistance. Scientists around the world are searching for practical uses for recent breakthroughs in superconductivity.

Researchers here at the Miyazaki test center of the Railway Technical Research Institute have already set a world speed record of 321 mph with an experimental version of the train, called ''MAGLEV'' for magnetically levitated vehicle.

A group of visitors recently were given a ride.

Passengers' backs were pressed against the seat, as happens during a jet plane takeoff, as the train accelerated rapidly.

Guided by a magnetic field instead of by rigid rails, the train swayed from side to side, leaving passengers with a sensation similar to being on an airplane buffeted by strong winds.

''We're still operating the magnets at only 93 percent of their capacity,'' explained Hajime Takagi, director of the test center. ''Within two months we hope to raise it to 100 percent, after which the sway definitely won't be a problem.''

Powerful superconducting magnets suspend the train four inches above hundreds of oppositely charged metal coils set in the base of its U-shaped guideway. They also keep the train centered in the guideway and propel it forward, through attraction to electromagnets along the guideway's walls.

Despite their strength, the superconducting magnets only need to be plugged into electricity briefly. Since they have no electrical resistance, current keeps flowing through their coils, creating the powerful magnetic field.

Magnetically levitated trains already are in service in Birmingham, England, along a route of less than half a mile between the city's train station and airport. However, the trains go only about 25 miles an hour.

High-speed versions are being developed by Japan Air Lines and by a West German consortium.

Japan was the high-speed railroad leader in developing its Bullet Train that can go 185 mph on standard tracks. France moilways, is the only system that uses superconductivity.

The MAGLEV's stronger superconducting magnets allow it to travel at higher speeds and levitate higher off its guideway, which Takagi says is safer in the event of ground shifts in earthquake-prone Japan.

But developers of other levitated trains, which travel about one-third of an inch above their tracks, say superconducting technology is still too complicated and expensive and not necessary for trains traveling at speeds that are slightly slower.

MAGLEV developers soon are likely to have longer tracks to test their technology. In January, Transport Minister Shintaro Ishihara announced that two new experimental lines will be built, one probably on the northern island of Hokkaido and the other possibly in central Japan.

The government is expected to provide $6.1 million for MAGLEV research in the fiscal year beginning in April. So far, a total of about $350 million has been spent on the system's development, officials say.

Researchers have turned to levitated trains because conventional trains running on tracks are limited to maximum speeds of about 185 mph or lower because their wheels begin to slip on the steel rails when they go faster.

But magnetically levitated trains also are being considered for slower- speed applications because of their low noise and potentially cheaper construction and operating costs. With no heavy undercarriages, the cars can weigh less than conventional trains, and their lack of contact with the road bed drastically reduces track maintenance.

Track construction for both the MAGLEV and Japan Air Lines system, called HSST, should cost less than that for the Bullet Train, officials for the two systems say.

The first commercial HSST system already is under construction in Las Vegas, Nev., and will link the city's downtown area with the ''Strip,'' about 4 1/2 miles away.

New breakthroughs in superconductivity should allow an eventual 30 percent reduction in the weight of the MAGLEV's magnets, researchers say.

Some technological problems remain, but Takagi says the biggest one is the superconducting magnet's effect on passengers' wrist watches.

Takagi said that at its strongest point the magnetic field inside the train measures about one-fourth the strength of a magnetic clasp on a woman's handbag.

But during a recent test run, one passenger's watch stopped for six minutes - exactly the length of the ride.

''There's no magnetic standard for watches, so we're not sure how low we have to reduce the magnetic field to eliminate damage to all watches,'' Takagi said. ''But we definitely plan to do so.''

End Adv Thurs PMs Feb. 11