Thomas had been at Guy Fawkes night celebrations in Lewes
A coroner has condemned the habit of so-called "train surfing" - when young people cling to trains as they travel between stations.
His comments followed an inquest on Thomas Clarke, 18, of Plumpton Green, East Sussex, who died after clambering on to the roof of a moving train as it headed towards a footbridge.
After drinking heavily and taking cannabis during the Guy Fawkes night celebrations in Lewes in November 2002, the teenager boarded a train at Lewes to travel home.
Toby Hutchins, a friend who was 16 at the time, watched him climb onto the window ledge of the train door and onto the roof - but was unable to stop him.
Heard a thud
By looking through the window and onto the roof, he was able to see his friend standing with his arms outstretched before walking along the roof to the rear of the train.
A moment later he heard a thud, as the train passed at 70 mph under the Novington Lane footbridge.
Thomas's body was discovered later that night. He had died instantly.
The train's driver, Philip Sistrom, told the court that he was increasingly worried by people "surfing" on trains - or clinging to the backs of carriages as they moved between stations.
The train with Thomas on its roof went under a footbridge at 70 mph
He said this was the first time he was aware of anybody climbing onto a roof.
Thomas's younger sister, who was not in court, had implied that her brother had done this before, telling the driver "he always climbs down".
Summing up, the coroner, Alan Craze, said this was the first time he had become aware of the trend of "surfing".
He described it as "very, very, deeply disturbing", and warned of the dangers of this "macho culture".
He said the trend needed the maximum amount of publicity, "to ensure nobody else ever does anything as reckless again".
The jury at Eastbourne Coroner's Court returned a verdict of misadventure.