A loose bolt was to blame for causing rush-hour misery to thousands of commuters earlier this month.
An investigation into the two- hour delay on the Downtown Line (DTL) on Oct 4 found that the hitch caused a platform screen door to dislodge from its frame at the Sixth Avenue station on DTL 2. The unhinged door came into contact with the side of an approaching train and shattered, preventing trains from passing through towards Chinatown.
Investigations found that the bolt on the fastener, which secures the door to the guide rail, had loosened over time.
The Land Transport Authority (LTA) and train operator SBS Transit (SBST) revealed details of the probe yesterday, but admitted that what caused the bolt to come loose remained a mystery.
An LTA spokesman told The Straits Times: "As the passing train had already dislodged and damaged the platform screen door, we were unable to determine conclusively how the bolt had loosened."
It added that platform screen doors at all DTL stations had been "rigorously tested" before the line was opened to passengers - they were opened and closed 2,000 times continuously. The fasteners will now be further secured with a self-locking nut and retaining pin. More checks will also be conducted on all DTL doors.
LTA and SBST have approached the French manufacturers of the doors - Faiveley Transport - to improve the design. Asked why it has decided to improve the platform screen door mechanism and if the original design was deemed inadequate, LTA said the move will "mitigate the risk of a re-occurrence".
It added that immediately after the incident, preventive measures were taken to secure the doors and apply a thread-locking adhesive to prevent the bolts from loosening.
The enhancement works are expected to be completed over the next two weeks. The platform screen doors for the upcoming DTL 3 will also use this improved design.
Doors at the Sixth Avenue station were last checked by SBST in August as part of routine maintenance and none was found to be faulty.
Following the Oct 4 incident, all doors along the DTL - including bolts and securing mechanisms - and along other rail lines were further inspected and found to be functioning normally.
The design of doors on the other rail lines will not be changed as they are different from the DTL doors and "have been operating well for some years without any report of loosened mechanical parts", LTA and SBST said.
Mr Lim Biow Chuan, a member of the Government Parliamentary Committee for Transport, said: "Accidents happen and we can't expect a perfect system."
While it is good that extra measures are being taken, it would be "worthwhile" to find out how the bolt came loose, to determine if it was a systemic problem or human error that caused it, he added.