A track laying machine works at the construction site of the section between Lhasa and Nyingchi of the Sichuan-Tibet Railway in Gonggar County of southwest China's Tibet Autonomous Region, Oct. 16, 2018. The Sichuan-Tibet Railway climbs from the Sichuan Basin several hundred meters above sea level to the "Roof of the World", at an altitude of more than 4,400 meters. (Xinhua/Purbu Zhaxi)

Workers lay railway tracks at the construction site of the section between Lhasa and Nyingchi of the Sichuan-Tibet Railway in Gonggar County of southwest China's Tibet Autonomous Region, Oct. 16, 2018. The Sichuan-Tibet Railway climbs from the Sichuan Basin several hundred meters above sea level to the "Roof of the World", at an altitude of more than 4,400 meters. (Xinhua/Chogo)

Workers lay railway tracks at the construction site of the section between Lhasa and Nyingchi of the Sichuan-Tibet Railway in Gonggar County of southwest China's Tibet Autonomous Region, Oct. 16, 2018. The Sichuan-Tibet Railway climbs from the Sichuan Basin several hundred meters above sea level to the "Roof of the World", at an altitude of more than 4,400 meters. (Xinhua/Purbu Zhaxi)

A track laying machine works at the construction site of the section between Lhasa and Nyingchi of the Sichuan-Tibet Railway in Gonggar County of southwest China's Tibet Autonomous Region, Oct. 16, 2018. The Sichuan-Tibet Railway climbs from the Sichuan Basin several hundred meters above sea level to the "Roof of the World", at an altitude of more than 4,400 meters. (Xinhua/Purbu Zhaxi)