Two trains hit each other on Sunday leaving 12 people injured.
Authorities have set up a casualty centre with 50 firefighters rushing to Fisherton Tunnel, near Salisbury, as one of the trains is on its side.
Network Rail tonight confirmed the crash unfolded when one of the trains derailed after hitting an object, and a second train then collided with it when signalling was affected.
About a dozen passengers are believed to have been hurt with reports they are “walking wounded”.
Student Callum Stedman, said passengers believed they would die and started calling their parents to tell them how much they loved them.
Callum, from Weybridge, Surrey, had been returning to Warminster School after the half term when the incident happened.
The boarder said: “The train was quite full. I was standing. We felt a jolt and everything went black.
“We all landed on each other and the train was at 45 degrees on its side.
“Then lights started coming on from people’s phones and we started looking around, you see people with broken noses and black eyes and blood dripping.