The signal was red at Sears Crossing.
Bringing his train to a halt, driver Jack Mills had no idea that it was fake.
And that red signalled danger as a ruthless gang of robbers prepared to strike…
Confused, his co-driver David Whitby climbed out of the train to ring the signalman, but suddenly he was attacked and thrown down the embankment after finding that the phone cables had been cut.
Seconds later, a masked man climbed into the train cab and knocked Mr Mills, 57, unconscious with an iron bar.
Then more masked men uncoupled the carriages, leaving just the engine and the first two carriages, as they stole the equivalent of more than £46million in today’s money.
Their £2.6million haul was indeed a fortune at a time when the average wage was less than £15 a week.
And the Great Train Robbery has become one of the most talked about crimes of the 20th century, with the men behind the masks, especially Ronnie Biggs after he escaped from jail, taking on an almost celebrity status.
The Royal Mail train had left Glasgow for London Euston on August 7, 1963, with staff on board sorting the post before its arrival.
Near the front of the train was the High Value Package carriage, where registered mail, mostly containing cash, was sorted. A Bank Holiday weekend in Scotland meant the total in the carriage was £2.6m, instead of the normal £300,000.
It was not long after the train passed Leighton Buzzard that the gang of 15 men struck. They’d stuffed a glove on the signal and lit the red light using a battery.
And with both drivers now out of action, the gang planned to drive the train a mile further to Bridego Bridge, where Land Rovers were waiting to move the cash to a nearby hideaway, a rented farmhouse in Oakley, Bucks.
But a man known as Old Pete, who’d spent months posing as a railway enthusiast, befriending staff on how to drive a train, realised the huge diesel train was too complicated.
The now infamous Ronnie Biggs had to rouse the driver to get the train going. “It has been rumoured that I was the brains of the robbery, which was totally incorrect,” he said years later. “And I’ve been described as the tea boy, which is also incorrect.”
When they finally reached Bridego Bridge, a human chain of robbers removed 120 sacks containing 2.5tons of cash. They told Post Office Staff to stay still for 30 minutes.
At their hideout they shared the cash out, playing Monopoly with real money. They’d planned to lie low for weeks, but got spooked by low-flying RAF aircraft.
They split the money, which was mainly in £1 and £5 notes, with Biggs receiving £147,000.
Criminologist Prof David Wilson, says: “In a sense this was so daring and people were fascinated by it. They had done something people would never dream of doing. There was an attraction and repulsion at the same time.”
Nine of the men were eventually caught and went to trial on January 20, 1964. Each was jailed for 30 years.
But the press and public were unaware seven more were still on the run. The robbery mastermind, Bruce Reynolds, lived it up in Mexico and Canada for 5 years, before being sentenced to 25 years and serving 10.
He was arrested by determined Scotland Yard detective Thomas Butler after returning to Britain when he ran out of cash.
Reynolds had turned to crime as a teenager after quitting his accounts job at the Daily Mail. He did time for theft before planning the train robbery which he described as “the Sistine Chapel ceiling” of his criminal career. He died in 2013, aged 81.
Ronnie Biggs was jailed but escaped from Wandsworth Prison in a furniture van 15 months later, and fled to Brazil.
His whereabouts only became known in 1974. “There’s a difference between criminals and crooks. Crooks steal. Criminals blow some guy’s brains out. I’m a crook,” he said.
For 36 years he led a playboy lifestyle in South America, with rumours of him having around 2,500 girlfriends, before he finally returned to the UK in 2001, after suffering three strokes.
Back in Britain, he was jailed and sent to Wandsworth Prison, but was freed in 2009, just two days before his 80th birthday, on health grounds. He died in 2013 aged 84.
Ronald ‘Buster’ Edwards is widely believed to have used the cosh to hit train driver Jack Mills over the head.
Mr Mills suffered brain damage and never full recovered from his injuries. He had to retire early and died in 1970, aged 64. Edwards, a former boxer and club owner spent three years in Europe and Mexico on the run.
He gave himself up in 1966 and was jailed for nine years.
After his release, he became a familiar figure working as a flower seller outside London’s Waterloo station.
Phil Collins played him in the 1988 film Buster, which was a box office hit.
In 1994, Edwards was found hanged in a garage at the age of 62. He had suffered from a drink problem and depression. His funeral cortège was accompanied by two wreaths in the shape of trains.
Four of the robbers were never brought to justice.
And almost 57 years on, the raid remains one of the most intriguing and talked about heists of all time.