A man killed when he apparently leaned from a train window and was hit on the head was a life-long railway fanatic who worked in the rail industry.
Simon Brown, from East Grinstead, West Sussex, died while travelling on the Gatwick Express in London on Sunday.
The 24-year-old first volunteered on the Bluebell Railway aged nine and was working as an engineering technician with Hitachi Rail Europe in Bristol.
Friend Reuben Smith said: "Railways were his life."
British Transport Police believe Mr Brown may have been leaning out of a train door window when he suffered a blow to the head.
Other passengers said he was looking out of the window when he was hit.
The incident happened at Wandsworth Common station at about 17:30 BST on Sunday, as the train was travelling to London Victoria from Gatwick Airport.
It has been reported to the Rail Accident Investigation Branch.
Mr Smith, a railway conductor who had known Mr Brown since the age of 12, has launched a fundraising page to help cover the costs of his friend's funeral.
Writing on the page, Mr Smith said: "Simon was a great friend of ours and a real light in the railway world.
"He was always smiling, cheerful and would bend over backwards to help anyone.
"There's now a massive hole in our close-knit community where he was.
"He has been taken from us at the age of 24 - it's far too young.
"Please donate. I want to help ease the burden of the cost of the funeral for his parents."
A transport police spokesman said: "The death is being treated as non-suspicious and the circumstances, including what caused the head injuries, are still being investigated."
Mr Brown's family have been informed of his death and a file is being prepared for the coroner.
A Gatwick Express spokesman said: "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."