The controversial Qinghai-Tibet Railway opened in 2006 and was the first train to access the Tibet Autonomous Region because of its altitude. At its highest point, the track runs at more than 16,000 feet (4,877 meters) above sea level, making it the world's highest altitude train. The trip from Beijing to Lhasa takes about 45 hours.
A large portion of the tracks on the Lhasa leg of the trip are built on permafrost, a permanently frozen layer of soil. Because operating the train generates heat, scientists and engineers had to sort out how to keep that heat from melting the ground underneath the tracks. By elevating the track and lining some areas with pipes full of circulating liquid nitrogen, they keep the ground temperature below freezing and maintain the tracks' stability.
The train winds up through the Kunlun Shan and Tanggula mountain ranges. At these altitudes, oxygen outside is scarce. As the train approaches its highest point on the Tibetan Plateau, it pumps additional oxygen into the cabins.
While the train's construction does mean more jobs, critics worry about its impact on Tibet and its culture.