At 4:15 pm on July 30, Yun's colleague found a subsidence under a subgrade along the Fengsha section after three rounds of inspection. After 15 hours of nonstop work, Yun and his colleagues fixed it on the next morning.

Just several hours later, Yun found that the water volume was excessive under a bridge. The rapid water was shaking the piers violently. K396 train from Wuhai, Inner Mongolia, to Beijing Fengtai Station was about to cross the bridge.

Yun called colleagues in Luopoling Railway Station immediately, asking them to call the train back to the station.