A 24-year-old man who died after reportedly leaning out of a train window may have been looking to see why the train was travelling slowly, a witness suggested.
The Gatwick Express passenger, from East Grinstead in Sussex, was on the high-speed service bound for London Victoria when the man was struck around 5.30pm on Sunday.
He was pronounced dead at the scene. The Rail Accident Investigation Branch has been notified and is investigating.
Witnesses on Sunday suggested the train may have been slowing down as it came into Wandsworth Common station, in south London.
The man, who was reportedly travelling alone, is believed to have then stuck his head out of an unlocked droplight window, usually used by conductors, prompting speculation he may have been looking at something.
At least one worker who witnessed the incident was signed off sick on Monday. A station source said: "A woman who works at the station is off sick today - she witnessed everything."
Questions were immediately raised about how the man could have been struck by an oncoming train, with some suggesting it must have been another object.
One witness, Kirstin Duffield, wrote on Twitter: "He was not hit by another train, it was on the west/verge side. He was not decapitated and nor did anyone scream."
But the station source appeared to contradict this, adding: "When they run through here nothing else gets close. It is likely he hit a train."
British Transport Police (BTP) confirmed officers were called at around 5.30pm following a report a man had received a serious head injury.
A spokesman said they believed the death was accidental and added that the circumstances were still being investigated. His next of kin have been informed.
Inspector James Tyrrell also contradicted earlier reports, which suggested the man may have been decapitated. He said: "We are aware of a number of reports on social media which say the man was decapitated, however this is not the case."
A spokesman for Gatwick Express, which is run by Southern railway, added: "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."