Before the last leg of the trip to Lhasa, the train stopped in Golmud early Monday to switch its standard engine for three powerful locomotives required to haul the train at high altitude.

The only signs of human habitation in the arid highlands south of Golmud were occasional small train stations and herders tending yaks.

After the train climbed above 13,000 feet, pens and bags of processed food burst due to the low air pressure. Laptop computers and digital music players failed, because moving parts in their disk drives are cushioned by tiny air bags that break at high altitude.

The railway is projected to help double tourism revenues in Tibet by 2010 and cut transport costs for goods by 75 percent. Until now, goods going to and from Tibet have been trucked over mountain highways that are often blocked by landslides or snow, making trade prohibitively expensive.

The New York-based group Students for a Free Tibet set up a Web site, rejecttherailway.com, urging the public to wear black armbands in protest of the project, which the group says "is a tool Beijing will use to overwhelm (the) Tibetan population."

"We reject the railway just as we reject China's illegitimate rule in Tibet," the site said.

Communist troops marched into Tibet in 1950, and Beijing says the region has been Chinese territory for centuries. But Tibet was effectively independent for much of that time.

The rail line is a decades-old dream for Chinese officials. But work began in earnest only in 2001, after engineers worked out how to stabilize tracks on permafrost.