The Gatwick Express passenger from East Grinstead was pronounced dead at the scene at Wandsworth Common station, south London, on Sunday.
Investigators believe the man, who has not been named, may have been leaning out of a train door window when he suffered a blow to the head at around 5.30pm.
Dismissing reports he may have been decapitated, the British Transport Police (BTP) said they have reported the incident to the Rail Accident Investigation Branch.
“The death is being treated as non-suspicious and the circumstances, including what caused the head injuries, are still being investigated,” a BTP spokesman said.
The family of the man have been informed of his death and a file is being prepared for the coroner.
A Gatwick Express spokesman said the train had been travelling to London Victoria from Gatwick Airport.
He continued: “The emergency services attended the scene at Wandsworth Common station and despite their best efforts, they were unable to save him. We send our condolences to his family.
“We are co-operating fully with British Transport Police and the Rail Accident Investigation Branch, who are investigating this incident.”
A spokeswoman for London Ambulance Service said they were called at 5.34pm and sent an ambulance crew, single responder and a car, and an incident response officer to the scene alongside an air ambulance.
They said they treated a person at the scene for a head injury, but they were pronounced dead by the doctor from the air ambulance.