A passenger is in a serious but stable condition in hospital after a train and a sewage lorry collided on a level crossing in Suffolk on Tuesday evening.
The train driver is suspected of having fractured vertebrae while another person was flown to hospital after the crash at Little Cornard near Sudbury.
Twenty-one others were also injured and the line is expected to be closed for several days.
The lorry driver, who is 38 and from Cambridgeshire, was arrested.
He is being held by Suffolk Police on suspicion of dangerous driving.
The seriously hurt passenger, a 58-year-old man, sustained life-threatening abdominal injuries, British Transport Police said.
Rail replacement buses have been set up for passengers.
The crash happened when the 1731 BST two-carriage diesel passenger train from Sudbury to Marks Tey struck the lorry.
The lorry was split open in the collision, spilling slurry over the scene, while the front carriage of the train derailed.
Locals said they helped bleeding and dazed passengers off the train and told of hearing a sound "like a bomb".
They said about 10 lorries crossed the line every day on their way to and from a nearby sewage works.
Network Rail said the train involved was a service run by National Express East Anglia with between 20 and 25 people on board.
A spokesperson said the level crossing was on private land with a locked gate on it and anyone wanting to go across needed to call the signaller to raise the gates.
The spokesperson said Network Rail had not received any calls prior to the crash.
Three men and two women remained in Colchester General Hospital overnight.
Their injuries consisted of broken ribs, back pain, chest pain and heavy bruising.
One has so far been discharged and the other four are expected to be allowed home on Thursday.
One other person also remained at West Suffolk Hospital in Bury St Edmunds overnight and the airlifted patient is being treated at Cambridge's Addenbrookes Hospital.
Detective Chief Constable Paul Crowther told BBC Radio 4's Today programme that British Transport Police was investigating the events before the incident.
"Our priority is to establish the evidence that is available from the train. Some of that we couldn't get overnight because of the position the train was in.
"There is a major engineering operation that's got to take place here, involving lots of equipment, with Network Rail. That's going to take some time to do."
A 130-tonne crane is being taken to the site to help clear the train from the line.
The lorry was removed from the line shortly after midnight.