New information has come to light regarding the cause of the derailment at Fisherton tunnel near Salisbury on Sunday 31 October 2021.
First reports said that a Class 158 train belonging to GWR, running as the 17:08 service from Portsmouth to Bristol Temple Meads, struck an obstruction in Fisherton tunnel as it approached Salisbury station. The train was derailed, which also damaged local signalling, and the South Western Railway 17:20 service from London Waterloo to Homiton, which uses the same tunnel, then hit the derailed GWR train.
British Transport Police then reported that 13 people were taken to hospital. One driver is said to have received life-changing injuries.
However, an interim report from the Rail Accident Investigation Branch (RAIB) suggests that, in fact, the two trains were on a converging course – their two lines came together at Salisbury Tunnel junction.
The South Western Railway (SWR) train should have been held at a red light while the GWR service passed in front of it, then it would be released to follow it through the tunnel. However, the SWR train continued past the red signal and collided with the GWR train at the entrance to the tunnel.
The RAIB report explains that Salisbury Tunnel junction allows the Up and Down Dean lines, which lead to and from Eastleigh, to merge with the Up and Down Main lines, which lead to and from Basingstoke. At the time of the accident, the GWR train (code 1F30) was using the junction to join the Down Main line from the Down Dean line, while the SWR train (1L53) was approaching the junction on the Down Main line from the direction of Basingstoke.
The movement of the GWR service across the junction was being protected from trains approaching on the Down Main line by signal SY31, which was at danger (displaying a red aspect). However, train 1L53 (the SWR service from London Waterloo) passed this signal while it was at danger, by around 220 metres, immediately prior to the collision occurring.
The impact of the collision caused the front two coaches of train 1L53 and the rear two coaches of train 1F30 to derail. Both trains continued some distance into Fisherton tunnel following the collision before they came to a stop. Thirteen passengers and one member of railway staff required treatment in hospital as a result of the accident, which also caused significant damage to the trains and railway infrastructure involved.
Preliminary analysis of data downloaded from the On Train Data Recorder (OTDR) fitted to train 1L53 shows that the driver initially applied service braking to slow the train on approach to the caution signal before signal SY31. Around 12 seconds 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 and warning system (TPWS).
However, 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.
EDITOR’S NOTE: Low adhesion can be due to many factors, but one that applies at this time of year is ‘leaves on the line’. Crushed fallen leaves leave a slippery film on the top of the rail that acts as a lubricant so that a train, even with all wheels locked, can slide a considerable distance.
The RAIB report will look into the causes of low adhesion, and how the operator could and should have handled the problem. A full report will be prepared.
Claire Mann, South Western Railway’s managing director, commented: “We welcome RAIB’s update on the scope and aims of its investigation.
“It is right that they look into all the possible causes of the lack of adhesion between the train and the track, and we are pleased their early assessment shows the South Western Railway driver reacted correctly to the signals by braking to slow the train down. We believe his actions went some way to preventing a much more serious incident and we wish him a speedy recovery.
“We will continue to work closely with the relevant authorities and our industry partners on all aspects of the investigation.”