UK: Wheelslide, ‘almost certainly a result of low adhesion’, was the most likely cause of a collision between Great Western Railway and South Western Railway diesel multiple-units at Fisherton Tunnel in Salisbury on October 31, according to the Rail Accident Investigation Branch.
‘Initial evidence indicates that the South Western train driver applied the brakes as it approached the junction and the red signal, but the train was unable to stop before passing the signal’, said RAIB Deputy Chief Inspector Andrew Hall on November 2. ‘This evidence suggests that the most likely cause of this was wheelslide, almost certainly a result of low adhesion between the wheels and the track.’’
RAIB set out what is known about the incident in an update issued on November 3. At around 18.45 on October 31, train 1L53, the 17.20 SWR service from London Waterloo to Honiton, collided with the side of train 1F30, the 17.08 GWR service from Portsmouth Harbour to Bristol Temple Meads. The collision took place at Salisbury Tunnel Junction, which is on the immediate approach to Fisherton Tunnel.
The junction merges the Up and Down Dean lines which lead to and from Eastleigh with the Up and Down Main lines which lead to and from Basingstoke. At the time of the accident 1F30 was using the junction to join the Down Main line from the Down Dean line, while train 1L53 was approaching the junction on the Down Main line from the Basingstoke direction.
The impact caused the front two coaches of 1L53 and the rear two coaches of 1F30 to derail. Both trains continued some distance into Fisherton Tunnel, before they came to a stop. One train driver and 13 passengers required hospital treatment, and there was ‘significant’ damage to the trains and railway infrastructure.
RAIB’s preliminary examination has found that the movement of 1F30 across the junction was being protected by signal SY31, which was displaying a red aspect. Train 1L53 passed this signal, while it was at danger, by around 220 m, immediately prior to the collision.
Preliminary analysis of data downloaded from the On Train Data Recorder fitted to 1L53 shows that the driver initially applied service braking to slow the train on approach to the caution signal before signal SY31. Around 12 sec after service braking started, the driver made an emergency brake demand. As the train approached signal SY31, and with the emergency brake still being demanded by the driver, a second emergency brake demand was made by the Train Protection & Warning System.
These emergency brake demands did not prevent the train from reaching the junction, where the collision occurred.
OTDR analysis indicates that wheel slide was present both when the driver applied service braking and after emergency braking was demanded. This was ‘almost certainly’ a result of low adhesion between the train’s wheels and the rails, RAIB said.
Investigation
RAIB’s investigation will seek to identify the sequence of events which led to the accident. It will also consider:
- the level of wheel/rail adhesion on the approach to Salisbury Tunnel Junction;
- the status and performance of the braking, wheel slide protection and sanding systems on 1L53;
- the behaviour of both trains during and following the collision;
- SWR’s policies relating to low wheel/rail adhesion;
- Network Rail’s policies relating to low wheel/rail adhesion and how they managed the risk of low adhesion in this area;
- the processes used to assess and control the risk of overrun at signal SY31;
- any relevant underlying factors, including any actions taken in response to previous safety recommendations.
Network Rail’s Safety & Engineering Director Martin Frobisher said low adhesion was ‘an issue that affects railways across the world and is something that we, and our train operator colleagues, work hard to combat, so that we can run trains safely and reliably throughout autumn, and why incidents such as the one in Salisbury at the weekend are incredibly rare. We will continue to work closely with investigators to understand what happened and what more we can do to help prevent this happening again.’
SWR said RAIB’s ‘initial findings indicate that the driver acted in an impeccable way in a valiant attempt to keep his passengers safe, staying at the controls throughout. We thank him for his actions and we wish him a speedy recovery as he continues to be treated in hospital.’
