The incident happened as Simon Brown was travelling on the Gatwick Express at Wandsworth Common station in south London.
Friend Reuben Smith has launched a fundraising page to help cover the costs of the funeral.
Brown, from East Grinstead, West Sussex, first volunteered on the Bluebell Railway as a nine-year-old and had recently taken up a position as an engineering technician with Hitachi Rail Europe in Bristol.
Mr Smith, a railway conductor, said: “Railways were his life. I have known him since we were 12. It is very sad.”
Writing on his fundraising page, Mr Smith said: “He has been taken from us at the age of 24 — it’s far too young.”
British Transport Police believe Brown may have been leaning out of a train door window when he suffered a blow to the head at around 5.30pm on Sunday.
Dismissing reports he had been decapitated, a spokesman said: “The death is being treated as non-suspicious and the circumstances, including what caused the head injuries, are still being investigated.”