Twenty passengers were injured — two seriously — after the 14:24 Southern service from London Victoria to Eastbourne derailed near Lewes shortly before 16:00 BST, prompting a major incident.
Three carriages left the tracks and several people were initially trapped, British Transport Police said, but all 150 people on board were later evacuated and those not needing hospital care were taken to a community centre.
Passenger accounts described carriages rocking violently, skidding and tipping onto their sides, with some of the rear coaches rolling down an embankment into a field beside the railway.
All lines through Lewes were shut, Southern told passengers not to travel across parts of East Sussex for the rest of the day, and journeys to Gatwick were expected to take at least 150 minutes longer.