A teenager may lose both his feet after being run over by a train, police said.
The 18-year-old is thought to have put his legs across the track at the Old Stoke Road level crossing in Aylesbury, Buckinghamshire.
He was struck by the 0603 Aylesbury to Marylebone train on Thursday and was taken to Stoke Mandeville Hospital with serious injuries.
British Transport police said they were not treating the incident as suspicious.
Firefighters, ambulance crews and British Transport Police were called to the scene.