Hundreds of MTR passengers were forced to abandon a train and walk along the tracks on Monday morning after a fault in the coupling between two carriages sparked the second tunnel evacuation in less than a month.
Firefighters were sent to the scene at Tseung Kwan O Station to assist with the evacuation of some 1,500 people. Two of the passengers felt unwell and were taken to hospital.
The incident, at around 8.30am, led to around four hours of service disruption to much of the Tseung Kwan O Line.
The government has told the MTR Corporation to submit a preliminary report on the incident within three days.
The Electrical and Mechanical Services Department said the railway company also has to submit a detailed investigation report within two months, accounting for the mishap and its causes, as well as suggesting improvement measures to prevent a recurrence of the problem.
The MTRC's chief of operations engineering, Nelson Ng, said there was a fault in the coupling connecting two compartments of the train as it entered the station. A safety device was triggered to stop the train, he added.
Ng said the train did not derail.
A woman surnamed Lo, who said she was standing at the connecting point in question, recalled hearing noises and said it felt like "something outside" had hit the train.
Passengers ran from the area, Lo said, which had "widened and bulged up". She said one of the carriages was left tilting slightly.
Another passenger who was aboard the train, a woman surnamed Chan, said she also heard some noises before the lights went out and the two carriages were plunged into darkness.
Chan said passengers waited for around half an hour inside the train, with some saying they felt unwell.
She added that it took her around five minutes to walk along the tracks to the platform at Tseung Kwan O Station.
The rail company's chief of operating, Sammy Wong, apologised for the incident and said the train would be inspected to find out what went wrong.
"Safety is the MTR Corporation's top priority. We will, of course, ascertain the cause when the train returns to the depot. We will also ensure other trains are safe before serving passengers," he said.
It was the second MTR tunnel evacuation in just over three weeks, after dozens of people escaped along the tracks when an accident on November 13 saw two sets of doors ripped off a train at Yau Ma Tei Station.
After legislators raised concerns about the safety of passengers entering MTR tunnels, the corporation said it would carry out a review of evacuation procedures.
On Monday, the chairman of Legco's subcommittee on railway matters, Ben Chan, said he may call an emergency meeting to discuss the latest incident.
"In just one month, two evacuations through the tunnels is very serious," he said.