James Hussey, who passed away in hospital aged 79, is said to have admitted the attack, which happened in 1963.
According to the Sun newspaper, he confessed to hitting the victim over the head to avoid taking the secret "to his grave"
Engine driver Jack Mills, 57, never fully recovered from his injuries and died seven years after the attack from leukaemia.
Hussey, known as "Big Jim", had worked for the 17 member gang as 'hired muscle', according to the newspaper.
He died in St Christopher's Hospice in Sydenham, South London.
'Deathbed' confession man recovers
23 Mar 2009
Kray made deathbed murder confession
25 Mar 2001
Mrs Biggs: why truth is stronger than fiction
05 Sep 2012
A source told The Sun he had "bottled it up" all these years, fearing another prosecution having already been sentenced to 30 years.
In recent weeks, the source claimed, his health deteriorated and he was moved from his home into a hospice.
"When he knew he didn't have long left, he decided to finally tell the truth," the unnamed source said. "He didn't want to take this secret to the grave."
The Great Train Robbers, including the notorious Ronnie Biggs, stole £2.6million from a Royal Mail train near Ledburn, Bucks, in 1963.
Mr Mill's son John, now 71, said it was "shocking" to hear a confession from Hussey "after all this time."
He added his father had not believed Hussey was the culprit.
"It devastated my family," he said. "I knew Dad would never be the same - and he wasn't."
The attack has previously been blamed on a mystery gang member known variously as "Mr Three" or "Alf Thomas".
James Hussey, a decorator, was sentenced to 30 years in prison for conspiracy to rob and armed robbery.