A man killed on a train may have been leaning from a window when he suffered a blow to the head, police have said.
The 24-year-old suffered serious head injuries while on board a Gatwick Express train at Wandsworth Common station at about 17:30 BST on Sunday.
British Transport Police said the death was not being treated as suspicious, and dismissed reports on social media that the man had been "decapitated".
The man, from East Grinstead, Sussex, was pronounced dead at the scene.
Police said the man's next of kin had been informed and they were still looking at what caused his injuries.
"Our investigation remains at an early stage, but initial enquiries suggest the man may have been leaning out of a train door window when he suffered a blow to the head," police said.
Insp James Tyrrell added: "We are aware of a number of reports on social media which say the man was decapitated as a result of this incident. However, this is not the case."
Kirstin Duffield said she was travelling on the train and stayed with the man as the carriage was evacuated at the station.
She said the man was "apparently travelling alone" and was "looking out of the window" when he was hit.
"There was no other train involved, it was the embankment side", she said.
British Transport Police have not confirmed whether this was the case or not.
The matter has been reported to the Rail Accident Investigation Branch.
A Gatwick Express spokesman confirmed there had been an incident on board a train that was travelling from the airport to Victoria.