Beijing, Oct. 15 (Reuters): China today announced the completion of the world’s highest railway, a controversial line that runs to its Tibet autonomous region.

The pan-Himalayan line, not yet in operation, climbs 5,072 metres above sea level and runs across Tibet’s snow-covered plateau, known as the roof of the world.

It has been criticised as an environmental and cultural threat to Tibet and is itself threatened by rising temperatures that could one day melt the frozen ground beneath it.

The official Xinhua news agency said $3 billion had so far been spent on the challenging 1,142-km final section whose construction across mountain ranges began four years ago.

“About 550 km of the tracks run on frozen earth, the longest in any of the world’s plateau railways,” Xinhua said.

Other feats included boring tunnels through ice as workers breathed bottled oxygen.

But even bigger challenges may lie ahead. The National Climate Centre said in June that rising temperatures would affect operation of the railway by 2050.

China says the line will promote the development of impoverished Tibet.

Critics say it will speed up migration from elsewhere in China and dilute Tibetan culture. Its construction has also been criticised for damaging the plateau’s fragile environment.