The South East Coast Ambulance Service has given its first major update on the incident, announcing two people have suffered serious injuries.
Nine others are less seriously injured.
Around 150 people were on board at the time of the derailment, it adds.
The ambulance service says it is continuing to respond to the "major incident", with other emergency services remaining at the scene.
"There were approximately 150 people on board the train at the time of the incident," it says.
"At this stage, we can confirm that two patients have sustained serious injuries and a further nine patients have sustained less serious injuries."
Patients were assessed and treated at the scene, it adds, with some transported to local hospitals for further treatment.
"A number of passengers who did not require hospital treatment have been taken to a local community centre, where they are being supported by partner agencies," it adds.