Snow and ice may have been a factor in a Range Rover coming off a main road and being hit by a freight train in a crash which left an 11-year-old boy critically ill, police said.
The collision happened on the A69 near Greenhead, Northumberland, at around 4.25pm yesterday on a straight section of 60mph road, and was thought to involve a mother driving her two sons.
The driver and a 15-year-old boy were not seriously injured, but an 11-year-old boy was flown to Newcastle Victoria Infirmary last night, where he remained in a critical condition.
There were early indications that, prior to going through a fence and on to the railway line, the Range Rover had been hit by another car which failed to stop.
Poor weather which swept across Britain may also have been a factor.
But police are keeping an open mind about the cause.
Chief Inspector Peter Reeve, of Northumbria Police, said: "The weather conditions were slightly icy and there was some light snow that may have been a factor but I am not ruling anything in and I am not ruling anything out.
"That may form part of our investigation."
The senior officer said the scene of the crash was still being examined, and the Range Rover would also be checked for any mechanical defects.
Police said the car was heading west along the A69 towards Carlisle when it veered across the carriageway, through a wooden fence and on to the railway track which runs beside the road.
The Range Rover was struck by a freight train heading towards Newcastle, and the car was extensively damaged.
Mr Reeve said: "We are appealing for anyone who was on the A69 in the Greenhead area at about 4.30pm yesterday to contact us.
"Even if they didn't witness the collision, they may have useful information that could help with the investigation."
Anyone with information is asked to contact police on 101 ext 69191.