Tourists are trekking from all over China to get a glimpse of the futuristic marvel, which is already hitting speeds of 431 kilometres an hour, faster than a speeding Japanese bullet train.

The German-built "maglev" train, which uses electromagnetic levitation to hover a few millimetres above the track, could spark a revolution in land travel. But skeptics say it could become another Concorde, technically dazzling yet financially impractical for most of the world.

Certainly it has proven a smash hit with the masses so far. In a three-day period last week, 8,000 passengers paid the equivalent of up to $54 each to ride the maglev -- even though it travels for a mere seven minutes before it has to slam on the brakes.

The train has been rocketing down its elevated track since early last month in a trial of what will become the world's first commercial maglev route: a 30-kilometre line between a Shanghai subway terminal and the Pudong International Airport.

On weekends and holidays, the train does 10 round trips every day. Regular tickets, promising the experience of "flying at zero altitude," are sold out a week in advance. Scalpers ask for double the official ticket price, often selling forged copies. Crowds gather below the elevated line to stare in wonderment.

With a bank of computer screens and keyboards, the train's cockpit resembles an Internet café more than a traditional locomotive. Even before reaching peak speed, it zips effortlessly past the cars on a nearby expressway. At top velocity, there is an audible hum and the train rocks a little, but the only slightly unnerving moment comes when it tilts as it banks into a curve.

"I felt a little dizzy on the curve, but it wasn't too bad," said Luan Jianping, a Shanghai man with a video camera around his neck who took his niece and nephew on the maglev last week. "It was like an airplane."

His 12-year-old niece, Lin Yijia, chattered excitedly as she boarded the train. "Maybe someday we'll have a high-speed train from Shanghai to Beijing and it will take only two hours, like a trip from home to the office," she said.

Promoters agree that the maglev could reduce the 1,300-kilometre Shanghai-to-Beijing journey dramatically from its current 15 hours. But it is far from certain, and would take up to $40-billion -- raising questions about the technology's cost and practical value.

Even in Germany, where the maglev was created and has been test-run at 550 kilometres an hour, it has never been put into commercial service. A proposed Berlin-to-Hamburg line was scrapped because of cost concerns, despite 30 years of public investment in the technology, and auditors have questioned whether two other planned lines would make economic sense.

In the United States, several maglev projects are under development, but most are behind schedule and none expected to be operational for a decade or more.

In Shanghai, though, the government was determined to build a showpiece project, regardless of the cost. It recruited a vast army of low-paid migrant workers to build the huge concrete pylons for the elevated track -- and they finished the project in barely a year.

"Three years ago, it was a fantasy," German Chancellor Gerhard Schroeder said at the train's inaugural ceremony in Shanghai last month.

"Few cities in the world can accomplish so much in so little time."

But the short airport route cost $1.8-billion, and critics say it will inevitably be a money-loser. When it goes into full commercial service at the end of this year, the ticket price is expected to be almost $10, meaning that a taxi will be cheaper, especially for two or more passengers.

Moreover, once they reach the end of the line, maglev passengers must lug their baggage down two flights of stairs to transfer to the subway to reach the city centre.

"I don't see much economic logic in a line that goes only halfway to the city centre," said Sam Crispin, a property consultant in Shanghai.

"Nobody needs to get to Pudong airport in seven minutes. Taking a taxi in half an hour is no problem," he said.