British Transport Police (BTP) do not believe the man, who is in his 20s, meant to harm himself when he suffered a “serious head injury” on Sunday (August 7) evening.
The high-speed airport service was bound for London Victoria and was about to arrive at Wandsworth Common Station, south London, when he was hit.
At least one worker who witnessed the incident at around 5.30pm has been signed off sick today.
A BTP spokeswoman said it is not thought to have been a suicide, adding: “It is believed to be an accident.
“The circumstances are still being investigated.”
The force is still trying to determine what the man’s head struck, with some reports suggesting it was another train.
The spokeswoman said: “That’s still being looked into at the moment. It is not quite clear but they are investigating what he was struck by.”
A station source said it was likely that his head struck another train.
They said: “When they run through here nothing else gets close. It is likely he hit a train.”
The source added that the fatality had left staff and witnesses traumatised.
They said: “A woman who works at the station is off sick today - she witnessed everything.”
The BTP spokeswoman said the victim’s age and where he lived would be announced today but the force would not be naming him.
She added: “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.
“The death is not being treated as suspicious and a file will be prepared for the coroner.”
The station remained opened today and showed no signs of the accident having taken place.
Inspector James Tyrrell disputed eyewitness’s accounts that the man was 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.
“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.”
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