Singapore Mass Rapid Transit (SMRT) said during media briefing on Tuesday (21 November) that the driver involved in the recent train collision at Joo Koon MRT Station is not to be blamed for the accident, because it was “very difficult and challenging” for him to understand what was happening and respond to the situation.
SMRT’s senior vice-president of rail operations for the North-South and East-West Lines (NSEWL), Mr Alvin Kek, said that it would have been difficult for the driver to grasp why the train ahead of it was not moving, since its drivers are “taught that there’s supposed to be at least one ‘protective bubble’” in place to keep a safe distance from trains near it.
The Land Transport Authority (LTA), operator SMRT and signalling system provider Thales have ascertained that the train collision on 15 November 2017 was caused by an unexpected disabling of a protective feature on the train that was hit, when it earlier passed by a trackside device at Clementi, which had yet to be modified for compatibility with the Communications-Based Train Control (CBTC) system.
They stated that when the train was travelling eastwards from Ulu Pandan Depot to Pasir Ris MRT station and then westwards from Pasir Ris MRT station to Pioneer MRT station, it was controlled by the old signalling system with the new CBTC system in passive mode. The CBTC system took over active control once the train entered the CBTC-controlled area west of Pioneer MRT station.
Investigation findings indicate that an abnormal condition on a train-borne CBTC signalling equipment disabled a feature designed to apply a protective “bubble” around the affected train to ensure safe distances between trains.
When such an abnormal condition occurs, the CBTC system is designed to immediately apply a second protective bubble (known as a “Non-Communicating Obstruction (NCO)”), to replace the first protective bubble. However, this second bubble was unexpectedly disabled when this train passed by a trackside device at Clementi which had yet to be modified for compatibility with the CBTC system.
They stressed that this did not affect the safe operation of the train as it continued to travel under control of the old signalling system along the EWL, eastwards towards Pasir Ris MRT station and then westward to Pioneer MRT station.
When the train arrived at Pioneer MRT station where it transitioned to the new CBTC system on the TWE, the train captain correctly detected the abnormal condition on the train-borne CBTC signalling equipment and reported it to the Operations Control Centre (OCC).
The OCC then directed the train towards the next station, Joo Koon MRT station to alight passengers before withdrawing the train from passenger service.
By design, the train was no longer allowed to move in Automatic Mode after the abnormal condition had been detected. As such, the train moved towards Joo Koon MRT station in Restricted Manual mode with a system-imposed speed limit of 18 kilometres per hour. The same speed limit was automatically imposed on the second train for safety reasons.
When the first train stopped at Joo Koon MRT station and the platform screen doors were manually opened, the “closed track” protection was automatically activated to prevent other trains from entering or leaving the platform.
Data logs from the trains have confirmed that the second train stopped about 36 metres behind the first train. Once the platform screen doors were closed, the “closed track” protection around the platform was automatically lifted to allow the first train to leave the station.
At this point, the second train started to move forward automatically as it could not sense any protective bubbles around the first train, which resulted in the collision at 8.20am.
Mr Kek explained that time was needed for the driver of the first train to shut the platform screen doors manually at Joo Koon after the passengers got off then he had to return to the driver’s cabin to drive off.
The LTA’s deputy chief executive for infrastructure and development, Mr Chua Chong Kheng, said, “The minute (the driver of the first train) lifted the closed-track protection after closing the doors, straight away the train behind would sense that it has a route to be able to move, so the second train started moving.”
In response to the accident, SMRT will be closing 19 stations on 10 and 17 December along NSEWL and will be ending services earlier for re-signalling work.