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Reynolds, who lived out his last years in Croydon, south London, had been in poor health in the last few months.
His son Nick Reynolds confirmed that he passed away in his sleep.
Bruce Reynolds said he wanted to "make his mark" when masterminding the Great Train Robbery - but said the infamous heist became his curse in later life.
Using inside information on the movement of valuables, the antiques dealer assembled a gang to raid a night train in Buckinghamshire in August 1963, with the group making off with £2.5 million in used bank notes.
The eventual death of train driver Jack Mills further blackened the heist.
While co-conspirator Ronnie Biggs spent nearly four decades on the run after escaping from prison in 1964, Reynolds evaded capture for five years, spending time in Mexico and Canada, before returning to England.
He was captured in Torquay in 1968 and jailed for 25 years but was released in 1978, alone and penniless.
Reynolds spent another three years in prison in the 1980s for dealing amphetamines.
He later said his part in the 1963 crime meant no-one wanted to employ him, legally or illegally.
Bruce Reynolds - the man who claimed to be the mastermind of the Great Train Robbery - has died just months before the 50th anniversary of one of Britain's most infamous crimes. He was 81.
Reynolds, who was born in London, fled to Mexico immediately after the robbery. Five years later, he was caught after returning to England and jailed for 25 years - but served nine.
The robbery took place in Buckinghamshire in August 1963.The gang held up the Royal Mail travelling post office, which ran between Glasgow and London.They stole more than two and a half million pounds - the equivalent of 40-million pounds today.
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There have been bigger heists since 1963. But in my primary school playground, the only game to play was Cops and Great Train Robbers.