The MTR Corporation said a detached collar in an energy absorption device caused the gangway between two carriages to "extend abnormally" in December, forcing the evacuation of around 1,500 passengers and triggering the partial suspension of the Tseung Kwan O line for four hours.

In an investigation report published on Friday, the railway giant said a collar responsible for securing the energy absorption device came loose, causing the device to dislodge and the gangway between the train’s sixth and seventh cars to extend abnormally on December 5 last year.

“The most credible cause of the detachment of the collar was the ineffective application of Loctite adhesive on its screw thread at its original build, thus the collar could not be locked in place permanently and eventually unscrewed itself over years of operation,” the report noted.

It stressed the gangway between the two carriages remained connected during the incident, and there was no misalignment of the train from the rail.

The railway firm said it has not identified similar problems in other trains, but will implement a series of improvement measures.

“We are implementing the improvement actions…including adding extra security to the collar by installing two additional lock pins … requiring the coupler assembly supplier to progressively replace the incident type energy absorption device with a design without the need of a separate collar, and exploring with the supplier the use of tools, such as technology, to monitor the operation of the device between overhauls for appropriate preventive maintenance,” operations director Tony Lee said.

The MTRC added that it will also enhance the dissemination of passenger information, and review the route arrangements for its free shuttle buses when incidents occur.