RAIB Publishes Interim Report on Salisbury Tunnel Junction Collision

The Rail Accident Investigation Branch (RAIB) has published its interim report into the collision between a GWR train and an SWR train at Salisbury Tunnel Junction that occurred on 31 October 2021.

Weather Conditions

On the day of the collision, there were strong winds and heavy rain in the Salisbury area. ~ 21mm of rail fell between 6am and 6pm and the wind guested up to 63km/h. The RAIB said “It is likely that the rain and wind were factors in the events leading up to the accident”.

The Collision

The 5:20pm SWR train (train reporting number 1L53) from London Waterloo to Honiton collided with the side of the 5:08pm GWR train from Portsmouth Harbour to Bristol Temple Meads at 6:48pm. The collision occurred at Salisbury Tunnel Junction on the immediate approach to Fisherton Tunnel.

Thirteen passengers and one employee required hospital treatment. Further, there was substantial damage to both the rolling stock and rail infrastructure.

The RAIB examined the railhead on the down main line over 3.2km on the approach to the collision site and found it to be contaminated throughout with a black deposit caused by crushed leaf material. It said “Samples taken from the area around signal SY29 and between there and the junction showed that the deposits were smeared and flaky and typical of the sort of contamination layers which exist after train wheels have slid over them”.

The SWR train was travelling within the permitted speed. The RAIB will look into the industry’s “guidance and instructions relating to reporting low adhesion and driving in such conditions”. According to the on-train data recorder (OTDR), the train wheels began to slide almost immediately after the driver applied the brakes. Further, the train’s wheel slip/slide prevention (WSP) system was active throughout the braking, but the train’s speed reduced only slowly.

The signal, which the SWR train passed, SY31, was at danger and the RAIB has found that the routing of trains across the junction was in line with normal signalling practice.

When the two trains collided, the SWR train was travelling at around 52–56mph.

Salisbury Tunnel Junction collision
SWR train involved in the Salisbury Tunnel Junction collision

The GWR train comprised two DMUs, each one made up of two cars. Upon impact, the coupling between the two DMUs came apart. The leading two cars, otherwise undamaged, came to a stop as their brakes applied automatically. The third car was derailed and forced to the left, against the tunnel wall. The final car was also derailed, its front left corner hitting the tunnel entrance.

Low-Adhesion Management

Network Rail has arrangements for how the risk of low adhesion should be managed during the autumn months. The collision took place in NR’s Wessex region, which issued its ‘Autumn working arrangements’ on 15 September 2021. The arrangements included railhead treatment trains (RHTTs). These deliver high-pressure water jetting and can also apply an adhesion modifier gel. These trains clean the railhead and improve friction between the rail and the wheel.

The Wessex route had a weekday schedule of two RHTTs to operate over each section of line every day “to maximise the chance of each site being treated at least once [every 24 hours]. Oner set of circuits run at weekends”.

The collision occurred on a Sunday and on that day an RHTT was scheduled to treat Salisbury Tunnel Junction on the down main line at 5:03pm. However, it had not passed through yet as it had been rescheduled as a result of planned engineering work between Southampton and Brockenhurst. Had the collision not occurred, it would have passed through at 11pm, which in turn would have meant that there would have been 36 hours between two RHTTs over the weekend.

The last RHTT to pass over this section of track did so at 11:06am on 30 October, ~ 32 hours prior to the collision. It delivered high-pressure water jetting. It was not required to apply adhesion modifier gel.

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