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 man was on a Gatwick Express train at Wandsworth Common station when the incident happened at about 17:30 BST.
British Transport Police said the death was not being treated as suspicious, and dismissed reports on social media that the man had been "decapitated".
He was treated for a head injury, but was pronounced dead by an air ambulance doctor, London Ambulance Service said.
Police said the man was in his 20s and officers were working to confirm his identity and inform his next of kin.
"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," they said.
Inspector 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."
The incident has been reported to the Rail Accident Investigation Branch.
A Gatwick Express spokesman confirmed there had been an incident on board a train which was travelling from Gatwick to Victoria.