A sign reading "different temperatures on the same train" is seen on the platform floor on Beijing Subway Line 6. (PROVIDED TO CHINA DAILY)

A preliminary investigation showed that emergency braking and a rear-end collision due to the snow caused the Beijing subway accident on Thursday, according to the Beijing Municipal Commission of Transport.

The slippery tracks resulted in a degraded signal, which caused the front train to break suddenly, and the train behind it was unable to break in time while coming down a slope, the commission said.

At around 7 pm on Thursday, the last two carriages of a subway train detached from the rest of the train between two stations along the Changping line. More than 30 passengers were injured

A total of 515 people were sent to hospital after the accident, with 102 suffering fractures. As of 6 am Friday, 423 people have been discharged from the hospital, 25 people are on observation, and 67 people are hospitalized for treatment. There were no reports of casualties.

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The Changping subway line in Beijing will operate normally on Friday except for sections affected by an incident on Thursday night in which two carriges detached from the train, the operator said.

To ensure the safe operation of trains at ground level or on elevated tracks during extreme weather conditions, trains will be operated in manual driving mode at lower speeds. Along the routes, bus services will be increased, Beijing Subway said in a notice.

At around 7 pm on Thursday, the last two carriages of a subway train detached from the rest of the train between two stations along the Changping line. More than 30 passengers were injured.

Videos and photos posted by netizens online showed that the connection between the two carriages had been severed. Some passengers broke windows to get fresh air, and some were shown walking along the tracks.

All evacuation work has been completed, and all injured people have been sent to the hospital. No one died, the notice said.

Later, the subway operator apologized in a notice, saying that it will make every effort to handle the aftermath properly.

"Passengers who left on their own during the rescue and evacuation but who later experience physical discomfort may contact the subway company at any time. We will bear the corresponding medical treatment and recovery expenses," the notice said.

Passengers are seen at Hujialou Subway Station during morning peak hours in Beijing, capital of China, Jan 3, 2023. (PHOTO / XINHUA)

Beijing has been experiencing heavy snow and cold temperatures since Wednesday, with a cold wave sweeping across North China. The subway incident happened during evening rush hour on a snowy day on an elevated track.

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The municipal government has established an investigation team to reconstruct in greater detail how the accident happened, to identify and hold accountable the parties responsible and to deal with injury and damage claims.

"We extend our sincere condolences and deep apologies to the passengers who were stranded, frightened and injured in this accident. We will make every effort to treat the injured and handle the aftermath," the notice said. 

"We will improve extreme weather operations and emergency response plans to ensure the safety of urban operations."