Network Rail and Greater Thameslink Railway are reporting that a passenger train has derailed near Lewes shortly before 16:00 BST, APA reports citing BBC.
Emergency services including South East Coast Ambulance Service, East Sussex Fire and Rescue Service and British Transport Police are at the scene.
A spokesperson for Network Rail and Southern Rail said: "We are aware that a passenger train has derailed in the Lewes area of East Sussex.
"Emergency services are on site and we will update with more information when we can."