An 11-year-old boy has been taken to hospital with potentially life-threatening injuries after the car he was in ended up on a railway line and may have been hit by a train.
Northumbria Police said the incident happened close to the A69 between Greenhead and Haltwhistle.
They say that they believe another vehicle was involved but left the scene. They calling on the driver to contact them.
A force spokesman said: "The car, for reasons yet to be established, crashed onto the railway line and may have received a blow from a passing train."
"The train stopped further down the line and does not appear to have suffered significant damage and no-one aboard appears to have been injured."
He said the boy was taken to the Royal Victoria Infirmary, in Newcastle, by air ambulance.
He said no-one else from the car appears to have suffered serious injuries following the incident which happened at about 4.25pm.
A British Transport Police (BTP) spokesman said: "BTP officers were called to the rail line in Greenhead, Northumberland, on Wednesday, December 5 following a report of a train striking a car.
"The incident was reported to BTP at 4.35pm and was also attended by colleagues from Northumbria Police.
"The train involved, a Carlisle to Tollerton freight train service, remained upright and on the tracks.
"A young boy was removed from the car, a black Land Rover, and has been air lifted to Newcastle Royal Victoria Infirmary.
"Three people are being treated at the scene for their injuries.
"Officers are working to establish the full circumstances surrounding the incident.
"The Rail Accident Investigation Branch (RAIB) has been informed."