The 76-year-old man, surnamed Hung, died after suffering altitude sickness during a trip to Tibet last month, according to the railway's chief operations director, Zhang Xiqing.

The incident, kept under wraps for about a month, came to light during a promotional trip for Hong Kong media. The man initially fell ill, Zhang said, after making the 26-hour trip, with his wife and a group of 23 others, from Qinghai to Lhasa, the capital of Tibet and the final stop on the railway.

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He subsequently checked into a Lhasa hospital where a doctor advised him to stay and recover for at least one month before venturing on the return trip. Against the doctor's advice, however, the man checked out and boarded the next train back to Qinghai.

But the high altitude reportedly triggered a heart attack early in the journey and the train was forced to make an emergency stop. The man was rushed to a nearby clinic where he was pronounced dead.

Zhang said the fatality in early August was the first in the two months since the opening of the railway, which has transported more than 270,000 passengers over the treacherous and cold 1,140-kilometer stretch since July 1.

Zhang also revealed that more than 550,000 workers and engineers fell prey to altitude sickness during construction of the railway, of which 14,500 required hospitalization. About 3,500 cases were deemed "critical" due to swelling of the lungs and brain, a known reaction to lack of oxygen.

One of the features of the train traveling across "the roof of the world" - where altitudes reach as high as 5,000 meters above sea level on the Tibetan plateau - is that more oxygen is pumped into the cars as the altitude increases. A Tibetan medicinal pill called Hong Jing Tian is also available for sale onboard.

Still, Zhang said, passengers should be fit and in good health and, once onboard, should move slowly to allow the body to acclimatize.

Speaking about security concerns, he said the railway's biggest fear was not hijackers but natural disasters that could block the tracks, such as snow, landslides and sandstorms.

In the colder areas, the operator recently discovered a number of cracks in the frozen soil supporting the tracks. Zhang said the company was still looking into the matter.

As for ecological impacts on the surrounding environment, the operator said it had witnessed few consequences, apart from the occasional collision with wild animals.

Reported "roadkills" to date include species of wild cows, horses and the famous Cashmere goat - indigenous to the high plateaus of Asia and best known for its luxuriously soft hair, which is often turned into sweaters.

Meanwhile, expansion of the railway network will continue, with mainland television station CCTV reporting that a new link to the railway, from Shanghai, would open in October.

That would create an eastern connection for the network, which already includes links at Beijing and Chengdu.

Construction on another supporting 270km rail line from Lhasa to Zigatse - the second-largest city in Tibet - will begin next year and finish by 2010.