Hong Kong's Beijing-backed Ta Kung Pao newspaper said: "Six trains were delayed along the line, affecting more than 4,000 passengers."

The daily Beijing News reported that the 16-carriage train from the southwestern city of Chongqing derailed near Co Nag Lake on Wednesday, about 400km northeast of the Tibetan capital, Lhasa. Only the dining carriage came off the tracks.

An equipment failure involved switches and signals, but trains were running normally five hours later and no one was injured, the report said.

High altitude

Meanwhile, the Xinhua agency citing a railway official reported this month that a 77-year-old tourist from Hong Kong on the railway, which reaches altitudes of more than 5,000 metres (16,400 feet), died from altitude sickness.

The man had been hospitalised in the Tibetan capital Lhasa, but discharged himself against the doctor's recommendation and boarded the train to Xining, in Qinghai province. He died at a station along the route, the report said.

Tibet has not escaped the wheels
of development

Three passenger lines are carrying tourists in pressurised cabins to Tibet from Beijing and the cities of Chongqing, Chengdu, Xining, Lanzhou. Oxygen is on tap if needed.

Chinese development

More than 960km of the railway was built at 4,000 metres above sea level and 550km in areas of frozen earth, which researchers fear could melt as winter temperatures rise in coming decades and affect operations.

China, which has ruled Tibet since its troops invaded the region in 1950, extols the railway as an engineering feat that will bring economic prosperity to the underdeveloped area.

But critics argue the line could endanger the region's fragile environment and Tibetans' unique cultural identity.