A pair of carriage doors fell off a moving train last month because of a lack of employee oversight and no preventive billboard maintenance by Hong Kong’s railway operator, an investigation report found on Thursday.

MTR Corporation’s investigative body said in the report that an outward leaning panel of a trackside billboard had hit and dislodged a pair of doors on a passing Kennedy Town-bound train as it approached a platform at the Causeway Bay railway station.

“The investigation concluded that the access panel leaned out and hit the first pair of offside train doors due to the failure of the two locks and two stoppers in holding the panel in position,” it said.

The accident on December 2 at about 6pm delayed thousands of commuters on the Island line, shutting down the route for about three hours and causing a knock-on effect on roads as people scrambled for alternative transport.

No injuries related to the incident were reported.

MTR Corp apologised to the public for the fiasco and pledged to improve measures to prevent any future incidents.

“Safety is always the top priority of MTR and I sincerely apologise to the passengers affected in the incident on behalf of the corporation,” said Tony Lee Kar-yun, the company’s operations director.

“We have been sparing no effort in eliminating the potential risk of such incidents from our network … We will also strengthen our maintenance regime as well as coaching and auditing the contractor to further enhance their staff’s quality of work and competency.”

The report said the movable access panel for changing the poster at the billboard was supposed to be held in place by two locks, with two stoppers for further protection.

“Before the incident, the latch of one of the locks had dropped, and the other lock was defective,” it said. “One of the stoppers was missing and the other one was found with one of its two mounting screws missing, resulting in the failure to stop the access panel from leaning out eventually after frequent rattling.”

The report said that workers for a contractor had failed to notice the defective lock, while the other was improperly attached because of poor understanding of how to fully latch it and its mechanism, adding that there was a lack of preventive maintenance from MTR Corp.

The accident on December 2 occurred when a pair of train doors collided with a leaning trackside billboard.

“While the defective lock was not detected by the contractor, for the stopper, the MTR Corporation did not explicitly request it to be included in the contractor’s checking procedures,” it said.

“The corporation had been relying on the contractor’s report of defects to arrange necessary repairs, and there was no specific preventive maintenance given to the locks and stoppers, particularly addressing their wear and tear.”

The investigative body said the contractor should enhance the procedures and training for its workers while MTR Corp would need to strengthen its maintenance regime for trackside billboards.

The rail giant has already removed all the access panels of the eight convertible trackside billboards and their associated parts to eliminate the risk of them leaning out.

Remote monitoring sensing devices with an alert function to ensure proper locking of about 300 advertising billboards with pulling front panels would be installed with additional locking indicators, it said.