With a cruising speed of nearly 270 miles per hour, it is the fastest passenger train in the world, but what makes the train truly extraordinary is what it lacks underneath its alloy chassis: wheels.

At a cost exceeding $1.2 billion, the Shanghai Transrapid line might be the most expensive 19-mile train route on the planet. By early next year it is due to become the world's first high-speed magnetic levitation or "maglev" train in full commercial operation.

The big question now is if it will also be the last -- for the foreseeable future, at least. A maglev train linking Baltimore and Washington, a concept pushed by Baltimore for more than 11 years, is one of three leading candidates for the first maglev line in the United States. But the project's future depends on $950 million in federal funds, on the support of the state, which would also have to provide money, and on whether private financiers believe the train can operate profitably.

The only working example anyone can consider when making decisions is the Shanghai Transrapid, which carried then-Chinese Premier Zhu Rongji and German Chancellor Gerhard Schroeder on its maiden voyage Dec. 31, less than two years after construction began. So far, the train is a popular tourist attraction, but the signals about its future are mixed.

The maglev's German designers had hoped to win a contract to build a line from Beijing to Shanghai, turning a 14-hour trip into a commute of three to four hours. But the China Railway Ministry appears inclined to consider proposals only from manufacturers of slower, wheeled trains, as critics question the economic and practical wisdom of entrusting a critical rail corridor to what they view as an unproven, expensive technology.

"The Australians considered the maglev between Sydney and Kampala. They gave it up," said Shen Zhijie, who retired in April as director of the High-Speed Track Office of the China Railway Ministry. "The Koreans also considered the maglev between Seoul and Pusan, and they also gave it up.

"The Beijing-Shanghai railway has to be for serious transportation. It's not for exhibition or tourism, and you cannot just argue theoretically."

China has a passion for railways and an appetite for grandiose public projects. The government is paying for the world's highest-altitude railroad (to Tibet), the world's largest dam and the world's third manned space program. With the Shanghai Transrapid, the country has the fastest passenger train. During daylight hours on weekends, the Shanghai Transrapid shuttles passengers between Shanghai's suburban airport and the city's urban outskirts, where Shanghai mass transit carries passengers the rest of the way into town.

An experiment

But for now the 19-mile, eight-minute trip remains mostly an experiment -- for the technology, the engineers who built it and the enthusiasts watching from far away, including Baltimore and Washington.

Above all, it is an experiment for Beijing's decision-makers, who appear to be leaning toward more conventional high-speed trains, like ones already in use in Japan and Europe, for the new Beijing-Shanghai line. That has to be troubling for the maglev's passionate adherents: If the maglev train can't succeed here, where the government can spend whatever it wants, and where more than a billion passengers a year take trains, then where can it succeed?

The Shanghai Transrapid begins each trip smoothly, gliding on an elevated track. On this particular weekday test run, a group of electrical workers is being rewarded with a ride.

As the train accelerates, the speed and time are displayed on digital screens at the front of each passenger car. The train reaches 100 mph in little more than a minute, and about 175 mph after two minutes.

An air suspension system, backed up by a secondary system of springs, keeps the train floating along without the bumps typical in wheeled trains.

But in the third minute, as the train approaches 250 mph, the passenger cars begin to wobble.

At the 3:21 mark, the train reaches the top cruising speed of almost 268 mph -- maglevs can go faster but not on this short route. The train moves through the air at such high speed that it creates a low whistling that could become unnerving on a longer trip. The engineers behind the maglev may have figured out how to get rid of friction with the tracks below, but there's no way to eliminate friction with the air.

The safety issue

The train's rates of acceleration and deceleration are carefully controlled, which means that people can stand and walk in the aisle at any point during the trip. Still, at these speeds, some passengers may become conscious of the lack of safety belts in a way they never would on typical trains.

The lack of safety belts was intentional -- the idea was that the train should look and feel as safe as any other.

"It doesn't need safety belts. There won't be a car crash, there won't be sudden braking and it will not turn upside down," said a high-ranking official with Shanghai Maglev Transportation Development Co. Ltd, who spoke on condition his name not be used. "It's not like the airplane or the car."

That theory may be tested on the maglev, which travels at speeds faster than some airplanes. At the trip's midpoint, a second maglev train rushed by on a parallel track at the same speed, producing a roar of air friction. On this day, the engineers were making sure that two maglevs could pass by each other safely at top speed.

Passengers have to adjust to the fact that the vehicle has no wheels and is not touching anything but air. They must remain confident the train won't suddenly spin out of control as it tilts and turns at 225 mph.

The train's German builders -- a consortium of Siemens AG and ThyssenKrupp AG called Transrapid International -- have been working to solve a series of safety and comfort issues. China's state news media have given few details about the technical problems, but they include concerns about rubber insulation overheating along the electrically charged track.

The Shanghai maglev official said the glitches were of no concern: "The damage caused by heat is no threat to safety. It's just like the clothing is old, but you can still wear it," he said. "There are also a lot of other problems, but none of them is a problem for the safety of operation.

"It's just like you buy a new television for your family and the channels are not tuned in perfectly. You make some adjustments, and it will be fine."

How it works

The maglev works thanks to a combination of magnets and electrical charges. Holding the train aloft is interaction between magnetic coils in the track and electromagnets on the underside of the train.

Electrical current flows through the track's coils, creating a magnetic field that pulls the train along as it floats less than a half-inch above the track. When the train is at rest, it may kneel down on the "guideway" to unload passengers.

In theory, the train can reach high speeds without requiring costly track maintenance. The friction created by wheels rolling on tracks has been eliminated.

Drawbacks

Unfortunately, the train's drawbacks are equally obvious: cost of construction and uncertainty about new technology. China expects to spend close to $16 billion to build a conventional high-speed Beijing-Shanghai line. The price for maglev construction might be substantially higher. Also, the maglev can operate only on its own tracks, unable to switch to the tens of thousands of miles of conventional tracks.

Another fundamental issue is the maglev's cost for passengers, though that may be a problem for other high-speed options as well. Per-capita income in China is less than $700 a year, but a round-trip ticket on the 19-mile Shanghai maglev costs $28. That is more than migrant workers and students pay for a one-way ticket for the 1,270-mile train trip between Beijing and Chengdu, the capital of Sichuan province.

"The maglev train is certainly not for migrant workers," said the Shanghai maglev official. "It's for businessmen and tourists."

That would be an issue in Maryland, despite Americans' vastly higher incomes. Maglev detractors in Maryland question a consultant's estimates that more than 30,000 people a day would pay a significant premium for the Baltimore-to-Washington trip. A one-way ticket could cost as much as $26, far more than the $7 charged by the much slower MARC train.

'Better investment'

Supporters argue that the maglev is a better bet than other high-speed options because it would have substantially lower operating costs.

"They might spend less money building [conventional] high-speed rail, but they will have a long-term higher cost because they will have a higher operations and maintenance cost," said Phyllis Wilkins, executive director of Maglev Maryland, which is funded through the city's economic development arm, the Baltimore Development Corp. "Maglev is a much better investment in the long run, and it's not just me saying that. It's people who have done studies who are saying that."

The potential appeal for China's well-to-do urban travelers may yet be enough to persuade Beijing to build another short track, in time for the 2008 Olympic Games, connecting Beijing's international airport with the city center at a cost of $700 million.

But it should be worrisome to America's maglev advocates that for longer routes, China remains hesitant to pursue the world's fastest passenger train.