The 17:31 National Express train from Sudbury to Marks Tey thought to be travelling at 60 mph collided with a sewage lorry at an unmanned level crossing on Bures Road on Tuesday evening.
A passenger on the train is in a critical condition in hospital and twenty others were also injured in the crash. Their injuries included abdominal pain, broken ribs and heavy bruising.
The two passengers who were seriously injured were trapped. They were taken to Colchester General hospital by air ambulance.
British Transport Police have arrested the 38 year old lorry driver on suspicion of dangerous driving and other rail safety offences.
Deputy Chief Constable Paul Crowther said Police Officers are currently searching the scene of the train crash to move the wreckage.
RMT Union leader Bob Crow commented: “Level crossings are a 19th century solution. It is high time for a commitment to separate rail and road traffic.”