The crash happened after a tanker collided with a train at a level crossing causing the back end of the train to derail.
An eyewitness who was interviewed on ITN news said that she could see "walking wounded" and described hearing a bang.
say that local police are calling it a "major incident."
reports that three have serious injuries and this includes the driver of the train.
Air Ambulances were dispatched to the scene. Firefighters and police were also in attendance
A statement from Network Rail
"At around 1735 on Tuesday 17 Aug there was a serious incident involving a road vehicle, namely a sewage tanker at a level crossing at Sewage Works Lane between Sudbury and Bures.
The road vehicle was struck by the 1731 National Express East Anglia service from Sudbury to Marks Tey. Initial reports suggest that three people were critically injured, including the train driver."
Mark Stone of Sky News, speaking from the scene of the accident, told Sky News
"It's moved now from a recovery to trying to work out exactly what happened