A train driver who was gravely injured in the Salisbury crash had tried to brake but suffered a “wheel slide” that could have been caused by leaves on the line, investigators believe.

       One of the trains involved in the collision on Sunday evening was “almost certainly” affected by “low adhesion between the wheels and the track”, which meant it was unable to stop at a red signal, the Rail Accident Investigation Branch (RAIB) said on Monday.

       Thirteen passengers were taken to hospital after a Great Western Railway service from Southampton to Cardiff collided with a South Western Railway service from London to Honiton as they both entered the Fisherton Tunnel in Salisbury at around 6.45pm on Sunday.

       Survivors described how they were thrown from their seats and had to climb out of broken windows after carriages tilted off the tracks and caught fire.

       The driver of the second train, a South Western Railway (SWR) service from London Waterloo to Honiton, had to be cut out of his cab and was left with “life-changing injuries”.

       The crash happened at a Y-shaped rail junction with both trains approaching the tunnel in the same direction but on separate tracks. One train struck the side of the other, causing it to derail, tipping its rear carriages on to their side.

       Investigators announced on Monday that early enquiries had shown that the driver of the second train did brake but was unable to stop before crashing into the other service.

       The probe will examine whether the train lost control due to a combination of wet weather and leaves on the line, a source said.

       Andrew Hall, RAIB deputy chief inspector, said the team had examined the track as well as analysing downloads from the trains' data recorders, electronic data from the signalling system and CCTV imagery.

       “From the initial evidence we have collected, we know that the passage of the Great Western train travelling from Eastleigh across Salisbury tunnel junction was being protected by a red signal. At this junction, trains coming from Eastleigh merge with those from Basingstoke, so the South Western service coming from Basingstoke was required to stop at that signal,” Mr Hall said.

       “Unfortunately, it did not stop and struck the side of the Great Western train at an angle such that both trains derailed and ran alongside each other into the tunnel just beyond the junction.

       “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.

       “This evidence suggests that the most likely cause of this was wheel slide, almost certainly a result of low adhesion between the wheels and the track. We are continuing to pursue this as a line of investigation amongst others.”

       The RAIB’s initial findings will be published later this week.