Low adhesion between wheels and track likely cause of Salisbury train crash
The initial findings of the investigation will be released later this week
The Rail Accidents Investigation Branch say that one of the trains involved in Sunday's (31st October) crash in Salisbury was "almost certainly" affected by low adhesion between the wheels and the track.
This meant it was unable to stop at a red signal.
The RAIB have said that their initial evidence suggests the driver of the South Western Railways service applied the brakes as it approached the tunnel, however it didn't stop in time.
It hit the stationary Great Western Railway train which caused both of them to derail.
They say they are continuing to pursue this line of investigation.
RAIB deputy chief inspector Andrew Hall said in a statement:
"A team of RAIB inspectors arrived on site on Sunday evening and were joined yesterday by additional inspectors and our support team. We are working alongside partner organisations including the Office of Rail and Road (ORR) and BTP. Our investigation is progressing well.
"Yesterday our focus was on evidence retrieval, we have been working with police scene officers to examine the train. We have also undertaken a close examination of the track and signalling in the area, and started to talk to those involved. Analysis of downloads from the trains' data recorders, electronic data from the signalling system and CCTV imagery is ongoing.
"From the initial evidence we have collected, we know that 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.
"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 wheelslide, 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.
"In consultation with other parties, we continue to work with the railway recovery engineers to ensure that the site is handed back in the shortest time possible. We intend to begin releasing parts of the site back to Network Rail later today."