At least 10 people were injured, three critically, when a lorry and a train were in collision on an unmanned rail crossing.
The driver of the lorry, which was believed to contain slurry, was arrested after the accident at Little Cornard, Suffolk.
Network Rail said initial reports suggested three people were critically injured, including the train driver.
A spokesman said: "At around 1735 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. There is also walking wounded.
"The crossing is a user-worked crossing with gates and telephone. The Network Rail signaller did not receive a phone call from the user of the crossing. British Transport Police are on scene and co-ordinating the response."
Suffolk police said the driver of the lorry was arrested.
The train was believed to be carrying more than 20 passengers, and around 10 of these were initially described as walking wounded.
The ambulance service said it had dealt with 18 casualties, of whom 14 were "walking wounded". Two people who had been trapped on the train suffered life-threatening injuries, and two others also had injuries which were less serious.
Superintendent Phil Trendall of BTP said the man was arrested on suspicion of a number of railway safety offences. He said 21 people had been injured, two of them seriously.
