The Southern Rail train from Victoria station in London to Eastbourne was carrying about 150 people when it went off the tracks outside the tourist town of Lewes.
Passenger Rob Bradley said the train began rocking as it picked up speed and the carriage he was riding in with his six-year-old daughter jumped upward and the brakes immediately engaged.
“Everything got thrown forward. My laptop, my AirPods, our bags,” Bradley told the BBC. “The train kind of skidded for quite a while before it stopped and only then when we turned around did we see that the whole back of the train — literally where we were sat — was open and there was smoke and dust pouring into the carriage.”
He could then see the car behind had flipped on its side and slid down an embankment into a lower field.
Bradley said he and his daughter were uninjured but he could see other passengers from one of the flipped cars with blood on their heads and clothing. Another passenger told him they had been cut when a window shattered.
Aerial footage showed firefighters and paramedics treating passengers in a field next to the train, with several ambulances and three helicopters standing by.
All passengers were evacuated within about three hours, the transport police said.
The cause of the derailment is under investigation. (AP)