A train driver is believed to have been unable to brake at a red signal before 'sliding' off the rails and ploughing into another service, according to an initial probe.
Fourteen people were hurt in the crash in a Salisbury railway tunnel - including a veteran South Western Railway train driver, who was left with 'life-changing' injuries.
Authorities investigating the incident announced on Tuesday that 'wheel slide' is believed to be a key factor, according to the first stage of their findings.
Passengers on the train described the moment of the impact in Fisherton Tunnel as like an 'explosion'.
Rail Accident Investigation Branch deputy chief inspector Andrew Hall said the SWR train had not stopped at a red signal before it ploughed into the side of a Great Western Railway (GWR) service.
Network Rail said in a statement alongside the findings that the experience would have been frightening for all aboard the trains, and added that 'low adhesion' is a phenomenon affecting railways worldwide.
Mr Hall said in a statement released on Tuesday that the passage of the GWR train travelling from Eastleigh across Salisbury Tunnel Junction was being protected by a red signal.
At this junction, trains coming from Eastleigh merge with services travelling from Basingstoke, so the SWR service coming from Basingstoke was required to stop at that signal.
Mr Hall continued: "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."
RAIB investigators are working with engineers to get the line back up and running as soon as possible, and are due to release parts of the site back to Network Rail later on Tuesday.
A SWR spokesperson said the RAIB’s initial findings showed the driver reacted correctly to the signals by braking to slow the train down.
The rail operator added: “We want to pay tribute to our driver who was injured on Sunday night. He is a deeply respected colleague, who has over 50 years’ experience of driving on this route and an excellent professional track record.
"All our drivers are regularly assessed to the highest standards and he has fully satisfied all requirements.
“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."
Claire Mann, Managing Director of South Western Railway, said: "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."
Passenger Angela Mattingly was travelling on one of the trains when they collided:
Network Rail’s safety and engineering director Martin Frobisher, said in response to to the initial findings: "Sunday’s accident was incredibly frightening for everyone involved and our thoughts are with everyone injured or affected in any way.
"Initial findings suggest that low adhesion played a key part in causing the collision. It’s 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.
He added: "We will continue to work closely with investigators to understand what happened and what more we can do to help prevent this happening again."
How do trains fail to stop? Analysis by ITV News Meridian's Mike Pearse
In the event of a signal failure, the system is designed to turn all lights red to warn of danger and stop services in the affected area.
Special safety systems are designed to stop two trains travelling on the same section of track.
Railways are also fitted with an automatic Train Protection Warning System (TPWS) that can apply brakes if a train passes a red signal.
Train passengers describe the moment of the crash to ITV News Meridian:
Even if the TPWS is triggered, slippery conditions can prevent a train from stopping safely.
In previous incidents, trains have been recorded slipping for more than a mile due to poor conditions on the tracks.
The train companies concerned told ITV News Meridian earlier on Tuesday it is still too early to conclude what happened while the probe is ongoing.
They added that their thoughts were with the injured and safety was the top priority as investigations continue.
The RAIB said it would release the full report of its initial investigation later this week, as authorities continue to speak to witnesses and examine CCTV and signalling data.