This is Daliang Tunnel, the world's highest tunnel for high-speed trains.
At an altitude of 3,608 meters, the tunnel is on the Lanxin high-speed rail line.
It passes through the Qilian Mountains, where there is snowfall nine months out of the year.
Track maintenance staff work night shifts starting at 1 a.m.
On numerous freezing cold nights they have walked and inspected tracks and machinery.
They are responsible for 140 km of tracks and the Daliang Tunnel, as well as six others.