A man has been killed after leaning out of a train window and being hit by another train, according to reports.
The passenger who was travelling on a Gatwick Express train was pronounced dead at Wandsworth Common station, south west London on Sunday evening.
London Ambulance Service said they were called to the station at 5:34pm.
"We sent an ambulance crew, a single responder and a car and an incident response officer to the scene alongside an air ambulance," a spokeswoman said.
"We treated a person at the scene for a head injury but sadly they were pronounced dead by the doctor from the air ambulance."
Police have said the man was in his 20s and are still working to confirm his identity.
Initial reports suggested the man had been decapitated, however Inspector James Tyrrell, from British Transport Police, said: "This is not the case.
"We are continuing to investigate the circumstances of the incident, which has been reported to the Rail Accident Investigation Branch (RAIB), and I'd urge anyone who saw what happened to contact us."
A spokesman for Gatwick Express said it first received reports about the incident on board one of its trains which was en route to Victoria from Gatwick at 5.45pm.
He said: "The emergency services are in attendance at Wandsworth Common station. We have no further information at this stage.
A BTP spokesman said the incident was not being treated as suspicious, and a file is being prepared for the coroner.
Lucie Walker, a 21-year-old student who was at the station told the Evening Standard: "I got off the train from Victoria and at once saw the ambulance helicopter that had landed nearby.
"There were lots of police around. The mood was very sombre."
Residents said they were shocked to see the air ambulance land on the common earlier in the afternoon.
One said: “I heard a man was hit by a train and heard the air ambulance land on the common. It's such a horrible thing to have happened.”
A local resident said: "We were coming back across the common when we saw the helicopter land. It was at about 6pm. There were lots of police and ambulances there, it was disconcerting."
Hours after the incident a police cordon remained in place at the entrance to the station as investigators continued to work at the scene.
Additional reporting by Press Association
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