SINGAPORE: A "software logic issue" with the new signalling system and a "confluence of failure conditions" caused the train collision at Joo Koon MRT station on Nov 15, 2017, the Land Transport Authority (LTA) said on Monday (Dec 18), as it completed investigations into the incident.
In a media release, LTA said its findings are consistent with initial findings presented at a media briefing last month.
LTA added that it has corroborated its findings with the final investigation report submitted by signalling system supplier Thales.
Echoing what was said at the Nov 21 media briefing, LTA said the collision arose from an "abnormal condition" on a train-borne CBTC signalling equipment.
"When this occurs, the CBTC system will, by design, apply a Non-Communicating Obstruction (NCO) protective 'bubble' around the affected train to ensure its safety. This was the requisite start state for the incident," LTA said. However, the train crossed "the only track point" that has not yet been fully modified for compatibility with the CBTC system.
The protective "bubble" was then "unexpectedly disabled" by the CBTC system, "as the software logic cannot correctly process a track point that has yet to be fully modified for compatibility with the CBTC system when running in passive mode".
"This is not an issue when the CBTC is running in active mode on the North-South Line and the Tuas West Extension (TWE) as they are fully on the CBTC system," LTA said.
LTA also said the failure conditions arose as a result of complications from operating two signalling systems concurrently on a single line.
"As a precautionary measure, operations on the Tuas West Extension (from Joo Koon to Tuas Link MRT stations) have been separated from the rest of the East-West Line (EWL, which operates on the legacy signalling system) since Nov 20," LTA said, adding that Thales has completed circuit modification works for the previously incompatible track point.
Thales is also modifying the software logic of the CBTC system to prevent future inadvertent disabling of the protective "bubble", LTA said.
LTA added that Thales will be setting up a CBTC simulation facility in Singapore to "strengthen testing processes" for the new signalling system in the North-South as well as East-West Lines.
The first phase of the facility will be completed by the first half of 2018 to enable off-site testing of the East-West Lines' CBTC signalling system before it is fully rolled out.
"When completed, this facility will be the first of its kind outside Paris/Toronto where Thales is based," LTA said, adding that the second phase will be complete by end-2018.
In response to queries, LTA told reporters that the new facility will be modelled after the actual CBTC signalling system on the North-South and East-West Lines, so as to enable the testing of both the system's software and hardware.
Its location "will be decided in due course", the authority added.
"This new facility will allow us to perform additional simulation tests which are tailored to the environmental and infrastructural conditions of the rail network in Singapore. The facility will also enhance our ability to test solutions for CBTC issues in a controlled setting without affecting train services," LTA said.
In response to reporters, LTA said it "reserves the right to take appropriate action against parties involved in the collision".
It noted that Thales has acknowledged its responsibility for the incident and has apologised to the public.
"LTA's immediate priority is to ensure the timely delivery of the multiple asset renewal works in the pipeline so as to improve rail reliability," said the authority.
"We will continue to work closely with SMRT and Thales to safely complete the upgrading of the signalling system for the North-South and East-West Lines as soon as possible.