British Transport Police (BTP) said in a statement: “Thirteen people were taken to hospital by ambulance, where they have received treatment for minor injuries. One remains there.
“Unfortunately, the driver of the train was more seriously injured and his injuries are believed to be life-changing.
“He also remains in hospital in a stable condition this morning, and his family have been informed.”
A “major incident” was declared by BTP on Sunday after a Great Western Railway service from Southampton to Cardiff collided with a South Western Railway service from London to Honinton as they both entered the Fisherton Tunnel.
Both trains, which were carrying 92 passengers in total, were travelling in the same direction and one train struck the other, causing it to derail in the tunnel.
Superintendent Lisa Garrett said: “The front few carriages remained upright, while the back tipped on their side.”
Detectives are still combing the crash scene for clues.
They are working closely with the Rail Accident Investigation Branch and the Office of Rail and Road to investigate how the trains came to collide.
Supt Garrett said officers are “keeping an open mind” and added “there has been nothing to suggest the train struck an object or that there was any significant delay between the trains colliding and one derailing”.
Asked whether “anything criminal” had been ruled out, she said: “We haven’t ruled out foul play at the moment. Until the investigation has actually concluded we cannot confirm why this happened.”