Earlier Monday the train, which has special pressurized cabins to combat the extreme altitude, crossed its highest point at the Tanggula pass on the Tibetan plateau, 5,072 meters (16,737 feet) above sea level.

However passengers, already 37 hours into their 48-journey, were mostly too lethargic and concerned about fighting off altitude sickness to mark the event around midday with too much revelry.

Many of the 800 passengers, 150 of whom were journalists, in fact were sleeping while others had stuck tubes into their nostrils to breathe in extra oxygen.

"Everything has been well planned," said passenger Xu Dongshan, 59, as he watched the odd lake pass by outside the window of his "soft-sleeper" first-class compartment while inhaling in oxygen from the tubes.

Xu, who was travelling with three friends, said he was feeling "rather well, and in good health."

Meanwhile, a waitress in the dining car was asleep on a cardboard box with the two tubes up her nose. "She is tired," her boss said.

However Xireyundan, a young Tibetan in a lower class "hard seat" compartment, declined the use of the oxygen tubes.

"I'm used to the altitude, it does not affect my health at all," he said.

Xireyundan is enrolled in a Beijing police academy and was returning home for the summer vacation.

The Tanggula pass is the peak of what is now the world's highest railway, surpassing Peru's Lima-Huancayo line, which reaches 4,800 meters.

Upon arriving at the western-Chinese outpost of Golmud in Qinghai province early Monday morning, three supplementary engines were added to help the train surmount the Himalayan heights.

The Qinghai to Tibet leg of the railway opened for service on Saturday with a train running the more than 1,100 kilometers (680 miles) from Golmud to the Tibetan capital of Lhasa.

A sign on board the train in Chinese, Tibetan and English informed passengers that it was forbidden to smoke between Golmud and Lhasa.

However the passengers who boarded the train from Beijing, including 40 foreign journalists accompanied by government minders, on Saturday night were the first ones to travel all the way from the Chinese capital.

Outside, groups of soldiers could be seen stationed every few kilometers (miles) along the railway line for at least 100 kilometers (63 miles) on the climb up to the Tanngula pass.

The military presence highlighted government efforts to ensure nothing went wrong on the trip, which Tibetan rights activist have warned marks the beginning of a flood of majority Han Chinese into the once isolated Himalayan region.

Previously Han Chinese could only get to Tibet on slow, uncomfortable and unreliable bus rides or on relatively expensive flights. Now, people can travel from Beijing to Lhasa on the train for under 50 dollars.

Besides the soldiers, only a few yurts and the odd herd of yaks gave witness to a human presence along the route that largely passes through a desert-like countryside.

Occasionally herds of Tibetan antelope were seen scurrying away from the railway as the train approached.

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