The first train from China's capital Beijing to Tibet's capital Lhasa has arrived after a two-day journey.
The train set off on Saturday following the inauguration of the Qinghai-Tibet railway, the world's highest.
The line boasts high-tech engineering to stabilise tracks over permafrost and oxygen pumped into cabins to help passengers cope with the high altitude.
China says the 1,140km (710-mile) line will bring major opportunities to a poor region.
But critics fear it will be used by China to assert its control over a contested border region.
They also say the railway line threatens not only the delicate Himalayan environment, but also the ancient Tibetan culture.
The first train from Beijing reached the Tanggula pass on Monday, where it climbed to 5,072m (16,000 feet), before beginning the descent to Lhasa.
Some passengers were affected by the altitude, and complained of feeling sick.
"Now we've reached the top, I feel sick and nauseous and have headaches," Wu Jia, 32, a Chinese tourist, told Reuters news agency.
According to a Reuters reporter on the train, passengers attached oxygen tubes to their noses and were forced to lie down.
China's official media has hailed the new Qinghai-Lhasa line as an engineering wonder.
China's President Hu Jintao, who watched the first train leave Golmud, in China's Qinghai province, for Lhasa on Saturday, called it a "magnificent feat".
Thin air
In parts, the train line has been built on bridges elevated above the most unstable permafrost.
Elsewhere, cooling pipes have been sunk into the ground to ensure it remains frozen to stabilise the tracks.
QINGHAI-TIBET RAILWAY
The train carriages have windows with ultra-violet filters to keep out the sun's glare, as well as carefully regulated oxygen levels with spare supplies to combat the thin air.
Zhu Zhensheng of the Chinese railway ministry called the new line a "major achievement" that will "hugely boost local development and benefit the local people".
But exiled Tibetan Lhadon Tethong said the railway was "engineered to destroy the very fabric of Tibetan identity".
"China plans to use the railway to transport Chinese migrants directly into the heart of Tibet in order to overwhelm the Tibetan population and tighten its stranglehold over our people," he said on a Free Tibet Campaign statement.
The Dalai Lama, Tibet's spiritual leader in exile since 1959, was more circumspect.
"The railway line itself is not a cause of concern for the Tibetan people," his spokesman, Thupten Samphel, said. "How it will be used is the main concern."
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