Sealed cars on refrigerated tracks in Tibet

The first passenger train to use the line is to leave Beijing on Saturday night and arrive in Lhasa, the Tibetan capital, 48 hours later on Monday evening.

The project links Golmud in Qinghai province with Lhasa. The $4.2 billion railway is billed as the world’s highest, crossing mountain passes over 16,500 ft high at speeds of 100 kmph.

About 80 percent of the railway ine is laid on potentially unstable permafrost, or frozen ground. To keep the permafrost stable, engineers sank pipes with cooling elements into the ground around the tracks to stabilise the embankments and ensure they stayed frozen. ‘‘It’s kind of like non-electric refrigeration,’’ Zhu said.

He said some 120 km of the rail line were also put on elevated bridges in spots where the permafrost was thought to be least stable. Global warming might threaten the rail line by softening the permafrost in about 50 years, but work is under way to prepare for such a possibility.