Software meant to fix a known problem in the East-West Line's new signalling system had been "inadvertently removed" and this led to the collision between two trains at Joo Koon station yesterday morning.
Sharing these details at a press conference 10 hours after the incident, the Land Transport Authority's (LTA) deputy chief executive Chua Chong Kheng said that it is investigating why this happened, together with operator SMRT and Thales, the French company supplying the signalling system.
Also present were Transport Minister Khaw Boon Wan, SMRT chairman Seah Moon Ming and SMRT group chief executive Desmond Kuek, but they were not part of the panel leading the press conference.
Mr Chua, who chaired the press conference, together with SMRT and Thales officials, revealed that earlier tests uncovered how an interruption in communications between onboard computers on a train would result in it being wrongly identified as a three-car train, rather than one with six cars.
To fix this on the East-West Line's new signalling system - which is used on the Tuas West Extension that opened in June - a software protection module was developed by Thales.
However, when a train passed by a faulty track circuit yesterday, this software module was "inadvertently removed". This caused it to be identified as a shorter, three-car train.
This subsequently led to an-other train behind "misjudging the distance" to the one in front, causing the collision, in which 27 commuters and two SMRT staff were injured.
Thales, which has installed signalling systems for metros in cities such as Hong Kong and Vancouver, described the glitch as unprecedented.
Mr Chua also said the same type of signalling system, which has been implemented on the North-South Line this year, has "proven to be working very well".
While the Tuas West Extension - a four-station, 7.5km-stretch that extends from Joo Koon station - is using Thales' new signalling system, the rest of the East-West Line is on an older 30-year-old system.
"We are in a stage where we are going through a transition of trying to implement the new signalling system... this (faulty) signalling circuit, as we understand from Thales, was going through the process of being changed," Mr Chua said.
He added that it was "too early" to say whether yesterday's glitch will derail original plans to switch the rest of the East-West Line to the new signalling system before the end of the year.
The new signalling system allows trains to arrive at up to 100-second intervals, instead of the current 120 seconds.
Yesterday's collision occurred as the first train was about to move off from Joo Koon station. Before that, it had stopped there to offload all passengers, due to a separate signalling problem.
While the train and platform screen doors were opened for passengers to get off, the signalling system would prevent another train from pulling into the station and hitting it, said Mr Chua. The second train initially stopped about 10.7m - the correct safety distance - behind the first.
Experts said yesterday's accident was a worrying one.
Assistant Professor Andrew Ng from the Singapore Institute of Technology's (SIT) engineering cluster said that if the trains were moving on the tracks, and the front one was mistaken as a three-car train, the rear one could misjudge the distance, potentially leading to a collision.
Assistant Professor Zhou Yi, also from SIT, said that suspending operations on the Tuas West Extension today is a good move and puts the safety of commuters first.
"We need to be very cautious moving ahead," Prof Zhou said, adding that SMRT and LTA should request for Thales to further open up its systems for a better understanding.
Mr Ang Hin Kee, deputy chairman of the Government Parliamentary Committee for Transport and an MP for Ang Mo Kio GRC, said it was "important to quickly identify the cause of the collision and to provide assurance to all commuters that the matter is being addressed".