Following in his father's footsteps, Cheng started work in 1995, when the mine was in its heyday. "At that time the mine was 170 meters deep, and it was as busy as a fair," Cheng recalled. "At night it was as bright as in daytime. We made six or seven trips in a shift."

Deng Yong, who works at Liushuquan station where coal from Sandaoling is transferred to other trains and then across the country, said of the stream train: "This giant piece of metal is pretty noisy, but not very powerful. It's very slow when it is dragging 50 cars."

Sandaoling used to be the largest opencast coal mine in northwestern China. Construction began in 1962, and it was completed and put into operation in 1970. The annual output reached 3 million tons at its peak.