For no more than a couple of minutes on 3 July 1938, Mallard thundered along at speeds that have remained unmatched by any steam locomotive for three-quarters of a century.
A handful of men in soot-stained overalls had pushed the roaring engine to 126mph, and in doing so had marked the pinnacle of steam power.
Today, 75 years to the day, Mallard will be reunited with her five surviving sister locomotives - two of which have been shipped from North America and restored - to commemorate her record-breaking run.
The event at the National Railway Museum in York is such a huge undertaking it will be "a once-in-a-lifetime celebration", organisers said.
"What we're planning is a major celebration - people will be coming from four corners of the earth," Anthony Coulls, the museum's senior curator of rail vehicle collections, said.
"The gathering of the six locomotives is the jewel in the crown really."
Despite its unique place in history, Mallard was one of 35 near identical A4-class locomotives designed by renowned engineer Sir Nigel Gresley - the man behind the Flying Scotsman.
The six survivors include Dominion of Canada, which now sits next to Mallard at the National Railway Museum after it was shipped from Montreal last October and restored especially for the anniversary.
And the Dwight D Eisenhower - another transatlantic expat - has also rolled back into York and been treated to a scrub-up for the occasion.
Union of South Africa and the Sir Nigel Gresley, will join them around the museum's Great Hall turntable for what organisers have dubbed The Great Gathering.
Bittern, the final survivor, travelled from London Kings Cross under its own steam on Saturday after it was granted special permission to make a celebratory 90mph run up the East Coast Main Line to York - 15mph over the normal limit for steam trains. It reached a top speed of 92mph and arrived on time.
Modelled in a wind tunnel, the Doncaster-built A4s' swooping art deco lines made them look like they were breaking records even when they were standing still.
But LNER's prime objective was simply to build a fleet of luxury express trains for its high-speed East Coast Mainline service, and one of the most long-standing speed records in history happened virtually by chance.
Mallard is thought to have been plucked from the pack because it was one of the newest locomotives and was fitted with a new performance exhaust, which Gresley wanted to test on the engine's home turf - the East Coast Mainline near Grantham.
"Mallard didn't really set out on that run to be a record-breaker," Mr Coulls said.
"They went to see what they could get out of it and it had a test car on the back which was noting down all the measurements.
"And they got the chance really on Stoke Bank.
"And they went for it.
"They knew that the only chance they could get to go that fast was on this part of the line.
"The record was made over not more than a couple of miles.
"It was quite a short distance but it was enough to get Mallard's place in the record books."
A July 1938 edition of the Railway Gazette rather reservedly reported: "A very remarkable speed record was set up by the London & North Eastern Railway on Sunday last.
"The record clearly shows the maximum to have been no merely momentary peak, but maintained at an even figure... "
Less widely reported was the fact that Mallard broke in spectacular fashion soon after it nudged 126mph and had to limp back to the workshop to have a bearing replaced.
Bizarrely, Gresley, who attended the record-breaking run, refused to accept the speed recorded by the dynamometer car, though he was confident the German speed record of 124.5 mph had been surpassed.
Others were less sceptical and the Mallard still wears a gleaming golden plaque on its side that reads: "On July 3 1938 this locomotive attained a world speed record for steam traction of 126 miles per hour."
But progress was not kind to the world's fastest locomotive.
Mr Coulls said: "Mallard's record was the pinnacle of steam and it was the swansong because these locomotives, within two years, were hauling troop trains.
"It's special because of what it did on that day in July '38, but for 20 years after it just settled down to being just another steam engine.
"By the end of their time, the A4s were dirty, they were unkempt and the glamour had gone."
As British Rail's 1955 modernisation plan began sweeping steam away, Mallard quietly ended its working life battered and downtrodden.
Diesel was coming, and rail travel would never be quite the same again.
It was withdrawn from service in 1963, while other A4s laboured on for a few years longer before making their final journey to scrapyards.
But Mallard was spared and treated to a full restoration by the National Railway Museum in the mid-1980s.
It is now one of the their biggest draws.
"Mallard carved its place in posterity in 1938, so it was always assured that something good would happen to it," Mr Coulls said.
"It is a '30s icon.
"It's not just an icon of the railways, it's an icon of style.
"It's like nothing else."