THE rails carrying China's showcase high-altitude train to Tibet began sinking into melting permafrost within months of its triumphant opening two years ago.
This was no mere technical setback for a pioneering engineering feat; for Beijing, it was essential for the Qinghai-Lhasa railway to function perfectly because it was a political project.
Conceived a century ago by Sun Yat-sen, the father of the revolution, the point of finally realising his near-impossible and hugely expensive dream was to set the final seal on China's benevolent "embrace" of Tibet.
That official narrative of unity and harmony between China and Tibet, exposed to the world as a sham as anti-Chinese resentment boils over in Tibetan monasteries and towns far across western China, is vitally important to Beijing for two reasons.
The first is that China has never really been able to control Tibet.
China's rulers have always felt the need to do so because Tibet's vast expanses command China's western flank and its borders with India, and also because of the huge influence of Tibetan Buddhism in Mongolia and throughout the northern border regions beyond the Great Wall.
Tibet has been a thorn in China's side ever since the Tang dynasty, when the two fought for two centuries before concluding, in 821, a treaty that, invoking the heavenly bodies, established "a great era when Tibetans shall be happy in Tibet and Chinese shall be happy in China".
That has never quite been the case. When the last imperial dynasty collapsed in 1911, Tibet swiftly declared independence. One of Mao's first acts after 1949 was to beat Tibet into line.
The second reason why Beijing needs Tibet to be convincingly pacified is ideological.
For many people, China has become an easier and freer place to live over the past 20 years, but it remains the case that the Communist Party cannot tolerate any belief system that even implicitly challenges its monopoly over "right thinking".
This is even more true today than it was, because with the demise of Maoism and, now, the jettisoning of Marxist-Leninism, the party lacks a belief system of its own to buttress its legitimacy.
Hence the party's pathological persecution of the eccentric but harmless Falun Gong religious sect and its increasingly harsh control of religious practice in Tibet, where Zhang Qingli, the Tibet party secretary sent there two years ago by President Hu Jintao, declared on his arrival a "fight-to-the-death struggle"' against the Dalai Lama.
The Chinese are paranoid about the Dalai Lama for essentially the same reasons that the rest of the world respects him: as the humbly persuasive spiritual leader of a religion whose lack of temporal power diminishes in no way the loyalty and love he commands.
He is the main reason why China's methods of ethnic colonisation, effective with other minorities, have failed in Tibet.
Not only is Tibetan culture too far removed from Chinese for assimilation to be feasible; it revolves around religious loyalties that the state cannot reach.
Because the Dalai Lama is at the centre of these loyalties, Beijing considers him a dangerously subversive political agitator.
They are appalled that he only has to make an address far away in India and his people obey; as when he advised Tibetans to stop wearing fur to save wild animals from extinction, and people rushed out to join public fur burnings.
Two years ago, rumours that he was returning swept Qinghai province and overnight thousands headed for the great monastery at Kumbum to greet him. To Beijing, this confirms what a danger he is.
The Dalai Lama talks about the Tibet problem in terms of "the identity of a people".
On this, Beijing agrees. It can end resistance in Tibet only by destroying Tibetan identity. It is deliberately swamping the population with Han Chinese and other immigrants, imposing "patriotic education" and Chinese-language qualifications for jobs, and stifling - other than as tourist exhibits - Tibet's customs.
The Dalai Lama seeks for Tibetans the autonomy to which they are lawfully entitled as an autonomous region of China.
But that would up-end Beijing's strategy. That is why China's leaders accuse him of inciting Tibetans to challenge, they say, the "stability of the state".
The first tumbrels rolled through Lhasa on Monday, hours before the deadline for "criminals" to turn themselves in or be hunted down, parading handcuffed prisoners with their heads forced down by soldiers.
Troops are in the neighbouring provinces of Sichuan, Gansu and Qinghai. Foreigners are being bundled out of Lhasa.
But Beijing will not keep the crackdown entirely secret. This is not only because it cannot, but because smashing dissent in Tibet would warn off the many other dissidents - farmers, migrants, the unemployed - tempted to try their Olympics-year luck.
Inside China, cracking heads in Tibet has up to now been pretty risk-free; few Chinese, shamefully, have much sympathy for Tibetans.
China's leaders are gambling that foreign protests will subside, as they have over Burma, in time to avert Olympic boycotts.
Have they guessed wrong? I remember Percy Cradock, then Margaret Thatcher's foreign policy adviser, sitting in Downing Street murmuring "when the dust has settled" when it was fresh blood, not dust, they were scraping off around Tiananmen Square.
To say "nothing must wreck the Olympics" after this would be much the same thing.