Twenty-one people were injured, one of them seriously, when a train collided with a sewage tanker on an unmanned rail crossing.
The front carriage of the two-car National Express East Anglia service, which was carrying more than 20 passengers, derailed after the crash at Little Cornard near Sudbury, Suffolk, on Tuesday.
The lorry driver, a 38-year-old man, was in custody after being arrested on suspicion of several railway safety offences.
Witnesses spoke of hearing a sound like a bomb explosion when the train and truck collided at 5.35pm, and then seeing dazed, bleeding passengers wandering around next to the track.
The lorry was also split open by the crash impact, spilling slurry across the scene.
Of those hurt, British Transport Police (BTP) said one person trapped on the train received life-threatening injuries and was in intensive care.
Five other people remained in hospital overnight with non-serious injuries, with 15 others described as "walking wounded" who also received treatment.
The driver of the 1731 Sudbury to Marks Tey service suffered a suspected fractured vertebrae, and was being treated at Colchester General Hospital.
All patients were taken to hospital - the majority to Colchester General Hospital, one or two to West Suffolk Hospital in Bury St Edmunds and one of the more seriously injured by air to Addenbrookes Hospital in Cambridge.
National Express East Anglia said there were about 20 passengers on board the train.