ABOARD THE BEIJING-LHASA EXPRESS — Chugging past shaggy yaks and fluffy clouds that look low enough to lasso, the train from Beijing to Lhasa makes its final climb into nosebleed territory pulled by three locomotives instead of the usual one.

Although some oxygen is pumped into the train cars as they roll through Tibet, the air inside has 30 percent less oxygen than it did some 2,100 miles ago, back in Beijing. As the express powers over its highest point — the 16,640-foot Tangula Pass — many on board feel it.

Dozens of passengers strap on oxygen masks, some experience bloody noses and a few lose their lunch. Pens spit their ink and potato chip bags expand until some burst their seams with the dramatic drop in atmospheric pressure.

For those looking for a novel way to visit one of the world's more remote corners, the new express train to Tibet offers an extraordinary trip. From the ubiquitous oxygen outlets to the vacuum flush toilets, from the flat-screen TVs in first class to the tracks anchored in the shifting permafrost, the "Sky Train" — as China calls it — is a marvel of modern engineering.

But the trip comes with political baggage. The Chinese government, which spent $4.2 billion to build the rail line, says that it will help invigorate Tibet's economy. But critics say it threatens to crush a Tibetan culture already weakened by 56 years of often harsh Chinese rule.

Many passengers on the first train from Beijing — which departed July 1 and arrived 48 hours later in Lhasa — seemed content to take in the views and overlook the controversy. They gazed out the train's windows — tinted to protect passengers from the harsh ultraviolet rays — mouths agape and eyes wide, drinking in the scenery.

Tibetan antelopes, wild donkeys, yaks and sheep grazed on wide open plains carpeted with spongy, bright green turf. In the distance, mountains rose up to the sky, their caps blindingly white with snow.

"It's hard to believe it's real. It looks like one of the wallpapers you can choose for Windows XP — but greener," said Li Changchun, a 25-year-old publishing executive from Beijing traveling by himself to Tibet for the first time.

Only very occasionally were there signs of human life — a herder's brown tent with a puff of smoke, a Chinese soldier standing guard along the tracks, a child in bright Tibetan dress waving madly as the 16-car train zipped past at 60 mph.

TROUBLE OVER RAILWAY

This pristine desolation is why many Tibetan rights groups and environmentalists have called on travelers to boycott the train. They say it will pollute the environment and threaten the wildlife. They fear it will be used to ship out vast quantities of minerals and other resources that were once prohibitively expensive to transport.

China says the line will help double Tibet's annual tourism income to $725 million by 2010. Chinese state media says the train will pull the "cork out of the bottleneck that has held the region's development back for decades."

Many average Tibetans seem conflicted over the railway. The exiled spiritual leader, the Dalai Lama, has said that it remains to be seen how the railway will be used, and whether it will bring real benefit to Tibetans.

Tashi, a 25-year-old Tibetan undergraduate from Beijing's Minorities University, took the train home for summer vacation.

"For me and for other students, the train is good because it's cheap," he said. "And a place that's closed forever can't develop. So for that reason too, I think it's good for Tibet."

But Tashi also said that political repression has increased in Tibet in the past five or so years. He said he's afraid of looking up articles about the Dalai Lama on the Internet and of talking about politics with people he doesn't know. He declined to give his full name for fear of recrimination.

BUILT ON ICE

On the environmental front, Beijing has earmarked $190 million for preservation projects along the railway and employed special technology to help protect the delicate permafrost that lies under much of the last third of the rail line.

Engineers designed sunshades, cooling pipes and loose gravel beds that conduct heat away from the ground to ensure the rail bed would stay frozen and stable.

The cooling pipes — resembling big metal golf tees — stick up on either side of the tracks for much of the journey. They use solar energy to turn liquid ammonia into a gas, repeatedly chilling the ground like a tiny refrigerator or air conditioner.

Passenger Yan Xiao, an engineering professor from the University of Southern California, said he was impressed by the design and service. "It's quite an achievement," he said.

The train's squat toilets might give some travelers pause, but it is cleaner and more spacious than the average Chinese train, and offers at least one handicapped facility with a seat-style toilet.

"It meets Western standards. It's fairly clean," said passenger Liu Yuejiang, a research scientist from Gaithersburg, Md..

But those considering a trip should go soon. Chinese engineers say global warming could threaten the permafrost, and the integrity of the rail line, in as little as 50 years.