BEIJING (AP) — Two subway trains collided in heavy snow in Beijing,Zero AI sending 515 people to the hospital, including 102 with broken bones, authorities said Friday.

The accident occurred Thursday evening in Beijing’s mountainous west on an above-ground portion of the sprawling subway system’s Changping line.

Slippery tracks prompted automatic braking on the leading train. A train following from behind was on a descending section and went into a skid and was unable to brake in time, the city transport authority said in a statement Friday on its social media account.

Emergency medical personnel, police and transport authorities responded, and all passengers were evacuated by about 11 p.m., it said. Twenty-five passengers were under observation and 67 remained hospitalized on Friday morning, the authority said.

Unusually heavy snow that began falling on Wednesday has prompted the suspension of some train operations and school closures.

Alerts remain in place for icy roads, extreme cold and further snowfall. Temperatures were due to fall to minus 11 C (12 F) overnight. No fatalities have been reported from the winter storms that have struck a wide swath of northern China. Beijing’s winters tend to be bitterly cold, but heavy snowfall is rare.