A maintenance worker checks equipment at the Xinguanjiao Tunnel, the world's longest plateau rail tunnel, along the Qinghai-Tibet Railway in northwest China's Qinghai Province, June 27, 2018. With a length of 32.6 kilometers, the tunnel was put into operation at the end of 2014. (Xinhua/Hou Deqiang)

A maintenance worker checks the tracks at the Xinguanjiao Tunnel, the world's longest plateau rail tunnel, along the Qinghai-Tibet Railway in northwest China's Qinghai Province, June 27, 2018. With a length of 32.6 kilometers, the tunnel was put into operation at the end of 2014. (Xinhua/Hou Deqiang)

Maintenance workers check the tracks at the Xinguanjiao Tunnel, the world's longest plateau rail tunnel, along the Qinghai-Tibet Railway in northwest China's Qinghai Province, June 27, 2018. With a length of 32.6 kilometers, the tunnel was put into operation at the end of 2014. (Xinhua/Hou Deqiang)

Maintenance workers check equipment at the Xinguanjiao Tunnel, the world's longest plateau rail tunnel, along the Qinghai-Tibet Railway in northwest China's Qinghai Province, June 27, 2018. With a length of 32.6 kilometers, the tunnel was put into operation at the end of 2014. (Xinhua/Hou Deqiang)

Maintenance workers monitor track data at the Xinguanjiao Tunnel, the world's longest plateau rail tunnel, along the Qinghai-Tibet Railway in northwest China's Qinghai Province, June 27, 2018. With a length of 32.6 kilometers, the tunnel was put into operation at the end of 2014. (Xinhua/Hou Deqiang)