At 1142 kilometres long the Qinghai-Tibet railway is not the longest train track on earth, but it was the most difficult to build.

The railway runs through a region so remote and treacherous it’s known as the ‘third pole’. The air temperature can sink to minus 40 degrees Celsius, the permafrost lies a half kilometre deep, and the altitude is so high, and the air so thin, that just being here can be fatal. Up against the forces of climate, temperature and topography, the engineers had to be innovative and determined to succeed.

A rail link across the Qinghai-Tibet plateau in the isolated south-west of China had been a national dream for over 50 years. Unsurprisingly all previous attempts have failed, for the plateau covers an area three times the size of France and reaches the staggering elevation of five thousand metres above sea level – half as high as Mount Everest.

Combined with the remoteness and the altitude of the Qinghai-Tibet region, the engineering team also had to contend with a terrain that changes dramatically between seasons.

Much of the region is covered in permafrost: rock hard in winter and melting in summer to produce a terrain of soft, unstable mud. The railway’s engineers needed to find a way of keeping the air and ground temperatures around the track at the same level all year round. Ironically in this frigid land it is the warm summer conditions, where the ground melts to a spongy mass, which are the most treacherous.

In some areas huge embankments are built to insulate the permafrost against the varying temperature. In others high-tech heat-radiating pipes filled with liquid ammonia are used to function like a refrigerator, drawing warmth away from the ground and replacing it with cold air.

But construction of the track is only one aspect of the Qinghai-Tibet project. Bridges, tunnels and stations are all built, and building them in this extreme environment requires a variety of innovative methods and techniques: from multi-layered insulation in tunnel walls (to, again, keep the melting permafrost from causing a collapse), to special oxygen facilities for the fitness of the 227,000-strong workforce.

However some things cannot be overcome. The Qinghai-Tibet railway runs through one of the most seismically volatile areas of the world. The railway crosses the point where the Indian and Eurasian tectonic plates meet. This volatile zone, where the plate’s up-thrust is pushing the Himalayas ever higher, is one of the most earthquake-prone places on the planet

But despite many obstacles, the Qinghai-Tibet railway has made it to completion: a national dream for half a century is now a reality. Against the odds of weather, terrain, seismology and altitude, the railway’s engineers have constructed a true Man Made Marvel: a railway across the roof of the world.