CBTC Implementation Part 2 – Train Collision

This article is Part 2 in a series of articles detailing the rollout of the Communications-based train control (CBTC) Signalling system on the North South Line (NSL) and East West Line (EWL) of the Singapore Mass Rapid Transit (MRT). Click here to read Part 1, which offers an introduction and background information with regards to CBTC, as well as challenges faced during the initial rollout stage where signal faults plagued the North South Line.

The CBTC upgrading project on the NSEWL was dealt a blow in the wake of an accident that occurred during the morning peak hour of 15 November 2017 where a moving train collided into the rear of a stationary train at the westbound platform of Joo Koon MRT Station along the East West Line (EWL), resulting in 36 injuries. The collision at Joo Koon was the second train collision in Singapore MRT’s history, after a 1993 accident at Clementi Station.

The accident dealt a further blow on SMRT Corporation and the authorities in the wake of a tunnel flooding incident and multiple faults plaguing the reliability of the North South Line and East West Line. In this article, we go through several technical details related to the accident and the subsequent impact of this accident which marked a turning point in the CBTC resignalling project.

This article builds on content presented in Part 1 (CBTC Implementation: A review) and readers unfamiliar with terminology presented here should read through Part 1, which offers background information on CBTC and the challenges of implementing it on the NSEWL.

The accident

Main article: Joo Koon train collision

Joo Koon Train Collision
First responders at the scene
First responders at the scene
Date / Time 15 November 2017,
Location Joo Koon MRT Station
Rail line   East West Line
Operator SMRT Trains
Type of incident Software failure in CBTC signalling system resulting in collision between two trains
Cause Software issues during switching of signalling systems
Train involved 2x Kawasaki-Sifang C151A trains
Injuries 38 (including 2 crew)

The Joo Koon train collision occurred on 15 November 2017 when a moving train collided into the rear of a stationary train at the westbound platform of Joo Koon MRT Station along the East West Line (EWL), resulting in 36 injuries. Both trains involved were Kawasaki-Sifang C151A trains and were operating on the Communications-based train control (CBTC) signalling system, which was recently installed at the time of the incident.


At the time of the incident, the Thales Seltrac Communications-based train control (CBTC) signalling system was gradually being introduced on the EWL to replace the legacy Westinghouse fixed-block signalling system.

Both legacy and CBTC signalling systems were in use on the same line at the time of the incident; however, the recently-opened Tuas West Extension was equipped only with the newer CBTC signalling system. As a result, trains travelling in both directions had to switch signalling systems at Pioneer. Between Tuas Link and Pioneer, trains ran fully on the newer CBTC signalling, and between Pioneer and Pasir Ris, trains ran on the older Westinghouse fixed-block signalling. All four models of rolling stock on the EWL, namely the Kawasaki Heavy Industries C151Siemens C651Kawasaki-Nippon Sharyo C751B and the Kawasaki-Sifang C151A, were equipped with both signalling systems.

Commencement of line testing was projected to commence the month after, in December 2017, to minimize the effect of rail disruptions on students. Similarly, for the NSL, live testing was conducted during the June school holidays. The CBTC resignalling project was already years behind schedule, having been delayed as other rail renewal work (such as sleeper replacement) were given priority.

Previous high-profile train disruptions which occurred during the testing of the CBTC signalling system on the North South Line (NSL) have also led to public grouses with the resignalling project. In return, the authorities urged for patience, with the promise of more reliable train services after the completion of rail renewal work.

CBTC: Active vs Passive mode

The CBTC signalling system governs automatic train operation in Active mode, which was used on the Tuas West Extension between Pioneer and Tuas Link. Since CBTC is the primary form of signalling governing train movement, it is said to be in Active mode.

Between Pasir Ris and Pioneer, the CBTC system operates in the Passive mode, which runs in the background to collect data for performance monitoring. This simulates actual CBTC signalling operation of trains while trains are still controlled by the old signalling system, allowing engineers to verify its performance, monitor system reliability and weed out any problems before trains switch over to being controlled by CBTC.

Prior to the accident, both passive and active CBTC systems on the East West Line were isolated. However, a day before the accident, signalling system provider Thales connected the passive CBTC system with the active CBTC system for the first time. This would contribute to the set of circumstances that led to the accident.

How the incident happened:


The CBTC system maintains a safe distance by constantly defining a safe envelope of operation for each train operating along the line, akin to a safety bubble. Using real-time communication through radio signals, trains are prevented from getting too close to each other by maintaining a separation distance between trains.

Likewise, the old signalling system also has protective bubbles which are defined using track circuits. Each physical circuit is about 200 metres long and is illustrated as a series of bubbles along the track, where no more than 1 train is allowed within each bubble at a time.

On the morning of 15 November 2017, at 5:38am, trainset 535/536 (a Kawasaki-Sifang C151A train) was dispatched from Ulu Pandan Depot in the eastbound direction and began revenue service on the East West Line, turning around at Pasir Ris. This train was controlled by the old signalling system, with the new CBTC system in Passive mode. When the train reached Pioneer, it switched over to Active mode as it departed towards the CBTC-controlled section of the EWL.

At the reception track of Ulu Pandan Depot, before joining the eastbound mainline, the train encountered an abnormal condition within the CBTC train borne signalling equipment which prevented it from communicating with the trackside CBTC signalling system. As a result, the First CBTC Protective Bubble was inoperative at the onset of the incident, as the CBTC requires constant communication between all trains to define the safe limits of operation for each train. However, the Legacy Protective Bubble remained active.

In response to the abnormal condition, the CBTC system interpreted the faulty train as Non-Communicating Obstruction (NCO) and applied a protective “bubble” around the train to ensure its safety. This is referred to as the Second CBTC Protective BubbleWhen the NCO is disabled, trains are only able to move in Restricted Manual (RM) mode, at a maximum speed of 18 km/h.


The NCO Protective Bubble (Second CBTC Protective Bubble) was disabled when the malfunctioning train passed by Point W210B (located just after the depot reception track merges onto the Eastbound EWL mainline), where an existing trackside device had yet to be modified for the new CBTC signalling system. By design, the a Temporary Speed Restriction (TSR) of 18 kilometres per hour was imposed by the CBTC signalling system. However, as the CBTC signalling system was only operating in shadow mode between Pioneer and Pasir Ris, the train continued to operate normally under the Legacy signalling system.

Train Captains are only required to monitor instruments related to the Legacy signalling system between Pioneer and Pasir Ris as it is the signalling system used by trains for that stretch. The CBTC signalling instruments are not visible to Train Captains on that stretch.

At Pioneer MRT Station towards Tuas Link, the train switched over to the Active CBTC signalling system. Due to the Second CBTC Protective Bubble being deactivated as described above, the train could only proceed at 18km/h in RM Mode due to the earlier signalling abnormality. Although the malfunction had existed since the train passed Point W210B towards Pasir Ris, the Train Captain could only identify the problem on his instrument panel after switching over to Active CBTC.

The Train Captain reported the abnormally to the Operations Control Centre (OCC) immediately at Pioneer, who made the decision to withdraw the train from service back to Ulu Pandan Depot (route indicated in green arrows). To accomplish this, the train was first directed to proceed forward towards Joo Koon (in RM Mode) to detrain passengers where it could be turned around.

As a safety precaution, the 18km/h Temporary Speed Restriction (TSR) was automatically imposed on the rear train for safety reasons. Also, as part of route protection, the rear train would not be allowed to catch up with the front train, so long as the train in front had not completed the route that was set for it to move from Pioneer station to Joo Koon station. Throughout the incident, the rear train would continue to operate in Automatic mode.

As the Passive CBTC system was connected to the Active CBTC system the day before, the malfunction was not self-corrected by the system. As both Active and Passive systems were previously isolated, a new protective bubble would have formed around the faulty train.

With the two CBTC Protective Bubbles disabled, the malfunctioning train was not protected by Automatic Train Protection systems. However, Train Captains and Train Controllers were taught that there would always be at least one protective bubble under the active CBTC system.

At 0818hrs, malfunctioning trainset 535/536 pulled into the Westbound platform of Joo Koon MRT Station (Platform B) to detrain passengers. Due to the onboard malfunction, Platform Screen Doors at Joo Koon Station were manually opened by the Train Captain via a switch (Emergency Screen Door Control) located at the Headwall of the Platform. Once the Platform Screen Doors were opened,  Closed Track Protection was applied, which indicates to the CBTC signalling system that Platform B of Joo Koon MRT Station was occupied.

At 0819hrs, the rear train, another C151A train (trainset 547/548), stopped approximately 36 metres away from the Tailwall of Joo Koon MRT Station Platform B. This was due to the Closed Track Protection that was in place for the faulty train at the platform. The stopping distance of 36 metres was confirmed with train logs and announced during the second press conference (it was estimated to be 10.6 metres during the first press conference).

At 0820hrs, after detraining passengers at Joo Koon Platform B, the Train Captain closed the Platform Screen Doors, which resulted in the Closed Track Protection being lifted to allow the faulty train to leave the station. As the faulty train did not have any Protection Bubbles around it, the rear train (547/548) accelerated forward automatically under the control of the CBTC signalling system which wrongly assumed that the track was clear to proceed (however, it was still limited to 18km/h by the TSR invoked by the front train).

10 seconds later, rear train 547/548 collided into the stationary  front train 535/536 before it could depart from Joo Koon station, at a speed of around 16 km/h, pushing it forward by the length of roughly two doors. There were 517 passengers on the rear train when it incident occurred. Passengers were detrained through the Train Captain’s door as the other passenger doors did not line up with the Platform Screen Doors.

38 people on the rear train were injured, including the Train Captain, an Assistant Station Manager and 6 Malaysian commuters. Most of the passengers injured in the incident suffered bruises from knocks and falls. The 38 injured people also included commuters who independently sought medical attention at hospitals after the incident.

The Train Captain of Trainset 547/548 had 1.5 years of driving experience on the North South and East West Lines at the time of the incident.

Fault Finding

The incident was a topic of substantial public discussion and cast the issue of rail reliability back into the spotlight. Much anger and pressure were levied at SMRT Corporation and the Land Transport Authority (LTA) in the wake of increasingly frequent rail faults in the preceding months. In particular, the Bishan tunnel flooding incident along the North South Line a month prior resulted in a public admission of failure as a result of maintenance lapses and falsification of maintenance records.

Thales, the firm supplying the new signalling system for the North South Line and East West Lines, said it had never encountered a glitch similar to the Joo Koon incident before, despite their involvement in many other brownfield resignalling projects on other metro lines overseas. It also accepted full responsibility for the incident and additionally assured that its CBTC signalling system is safe and secure for operations.

The Train Captain of the rear train was absolved of responsibility in this incident. It was reasoned that 10 seconds it took for the train to hit the one in front was not enough time for Train Captain to react, according to SMRT. Given that the train was operating in Automatic mode, and existing driver training dictating that there will be at least one protective bubble in place at any one time, he would not be able to correctly access the situation and take immediate corrective action given the circumstances.

Detailed Investigation

The root cause of the Incident was identified as a software logic issue with the CBTC system which was not configured to cater for the failure conditions that occurred on 15 November 2017. Further extensive tests and analysis by LTA, SMRT and Thales, including on-site reenactments, confirmed that a confluence of the following failure conditions contributed to the Incident:

  1. Train 3535/3536 had developed an abnormal condition with its train-borne CBTC equipment which prevented it from communicating with the trackside CBTC signalling equipment. This prevented the CBTC system from tracking the location of Train 3535/3536 accurately. When such an abnormality occurs, the CBTC system will, by design, apply an NCO protective “bubble” around the affected train to ensure its safety. This was the requisite start state for the Incident.
  2. Train 3535/3536 crossed over track point W210B which was the only track point that had not yet been fully modified for compatibility with the CBTC signalling system for trains operating in CBTC passive mode. Because of this, the status of point W210B could not be properly detected by the CBTC system, and was registered by the CBTC passive mode as ‘moving’ indefinitely
  3. The CBTC signalling system configured in passive mode had not accounted for the possibility of a point registering as “moving” indefinitely. As a result, the CBTC signalling system did not propagate the NCO protective “bubble” for Train 3535/3536 through point W210B.

Analysis and tests conducted by Thales on the ‘moving’ point scenario have also confirmed that trains operating in the CBTC active mode, such as those on the North-South Line (NSL) and TWE, would have correctly handled the ‘moving’ point scenario and propagated the NCO across the point. Thus, operations in CBTC active mode on the NSL and TWE continue to be safe.

Remedial Actions

Long Term Closure

Since 20 November 2017, operations on the TWE (which operates on the CBTC signalling system) from Joo Koon to Tuas Link MRT stations have been separated from the rest of the EWL (which operates on the legacy signalling system) to minimise the complications of operating two different signalling systems concurrently on a single line. This will eliminate the possibility of a recurrence of the events of 15 November 2017. The separation will continue until EWL re-signalling works are completed next year and the entire EWL, including the TWE, can be operated continuously on the new CBTC signalling system.

Changes to train services are detailed at Suspension & Changes to Train Service on Tuas West Extension, and shuttle bus arrangements are detailed at Joo Koon—Gul Circle Free Shuttle Bus Service.

System Modification

Thales completed the circuit modification at point W210B on 16 November 2017. Thales will also modify the software logic to ensure propagation of the NCO protective “bubble” across points regardless of the status of the point for both the CBTC active and passive modes. This will prevent future disabling of the NCO protective “bubble”

Immediate Aftermath

Train service between Boon Lay and Tuas Link were suspended until the evening as recovery works were carried out. As a precaution to prevent a similar incident on the North South Line, as it is operated using the new CBTC signalling system, the number of trains deployed during the evening peak hours on 15 November 2017 were reduced to allow for a larger gap between trains.

The Days Ahead

Short-term suspension of Joo Koon—Tuas Link

Following the incident on 15 November 2017, arrangements were made by LTA, SMRT and Thales to facilitate the investigation to the incident. From 16 to 19 November 2017, train services from Joo Koon—Tuas Link were suspended, and free bridging bus services were provided between Joo Koon—Tuas Link. Changes to train services are detailed at Suspension & Changes to Train Service on Tuas West Extension.

Long-term closure of Joo Koon—Gul Circle

From 20 November 2017, train services resumed between Gul Circle—Tuas Link, but services between Joo Koon—Gul Circle remain suspended. The decision was made to isolate the two different signalling systems ‘for about a month’ until a solution to the switching of signalling systems was found. This arrangement was done to separate the old signalling system-equipped section between Pasir Ris and Joo Koon from the new signalling system-equipped section between Joo Koon and Tuas Link, hence no longer requiring trains to switch signalling systems at Pioneer (Tracks between Pioneer and Joo Koon can be operated on either the old or new signalling system).

However, on 21 November 2017, an announcement was made to extend the closure until mid-2018 when resignalling works on the EWL were projected to be complete, so as to fully prevent a similar incident from occurring again. In the meantime, a Free Bridging Bus service was offered for commuters between Gul Circle—Tuas Link during rail operating hours.

Changes to train services are detailed at Suspension & Changes to Train Service on Tuas West Extension, and shuttle bus arrangements are detailed at Joo Koon—Gul Circle Free Shuttle Bus Service.

Shortened operating hours for accelerated maintenance

To accelerate the CBTC resignalling project on the EWL as well as additional engineering works, the operating hours will be shortened for sections of the EWL and NSL. This would comprise early closures, late openings and full day closures, with Bridging Bus Services offered to commuters during periods of closure.

Works started in December 2017 with shortened operating hours implemented along Tuas Link—Tiong Bahru (EWL), and Jurong East—Bukit Gombak (NSL) on various Fridays, Saturdays and Sundays. Detailed arrangements are outlined in Shortened Operating Hours on East West & North South Lines (Dec 2017).

Train network closures

During the tendering process, Thales assured that no network closures would be required for the resignalling project, given the inconvenience caused to commuters and lack of alternative public transport modes during a closure. Despite this, shortened operating hours were implemented to accelerate the resignalling project especially for the East West Line, and hence re-open the broken link between Joo Koon—Gul Circle as soon as resignalling work was complete.

As buses have substantially lower capacity than trains, a large volume of buses and crowd control resources would be needed to ferry commuters. In fact, it was announced that the first Sunday full-day closure (on 10 December 2017) involved 350 buses and 700 Bus Captains. The bus arrangements are expected to cost a lot of money, and includes other costs such as part-time crowd marshalls, auxiliary police presence, rental of crowd control barriers, catered food, etc.

The cost of this operation will be borne by the Government and rail operator SMRT. Buses bridging Joo Koon and Gul Circle are free, but those plying along stations affected by the shorter hours will charge equivalent train fares for the distance.

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