Construction on the last two of the 47 tunnels on a 435-km railway linking Lhasa and Nyingchi in southwest China's Tibet Autonomous Region was completed Tuesday, marking huge progress for the mammoth project.
The Mainling Tunnel, located in the city of Nyingchi, is 11,560 meters long with an average elevation of 3,100 meters above sea level and a maximum burial depth of about 1,200 meters.
It is believed to be one of the most difficult tunnels ever built.
The 8.7-km Zagar Tunnel in the city of Shannan traverses seven faults of complex geological structures.
The completion of the two tunnels has increased the total tunnel length to 216.5 km, almost half of the railway length.
Meanwhile, 75 percent of the Lhasa-Nyingchi railway is bridges and tunnels, and over 90 percent of the railway is on the Tibetan plateau at over 3,000 meters above sea level.
As Tibet's first electric railway, it has a designed speed of 160 km per hour.
Over 20,000 builders have been working on the railway since the end of 2014. It is expected to be completed and put into operation in 2021.