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The A4 “Pacifics”: The Fastest Steam Trains in History

Written by Matthew Copes 


In early July of 1938, a sleek two-tone blue steam locomotive known affectionately as “Mallard” sat idling on the standard gauge tracks of the London–Edinburgh East Coast Main Line approximately 50 miles (80 km) northeast of Birmingham. 

Nine feet (2.7 m) wide, more than 13 feet (4 m) tall, nearly 72 feet (22 m) long and packed to the gills with coal and water for the historic event, the futuristic looking locomotive tipped the scales at a whopping 165 tons (149,700 kg) not counting the weight of the six coaches and dynamometer car behind it, the latter of which would determine how fast the train was traveling and how much power its engine was producing.  

Then, reacting to a rush of superheated steam from the boiler, three massive cylinders measuring 18.5 inches (47 cm) across and 26 inches (66 cm) from top to bottom located deep inside the train’s mechanical bowels began thumping methodically.    

Emitting intermittent puffs of smoke and steam from its dual exhaust stacks and abundant pressure release valves, the train suddenly lurched to life as wide-eyed spectators looked on with anticipation.

Slowly at first, the mighty machine steadily gained speed heading south toward London more than 280 miles (450 km) away.  

As pressure rose toward the boiler’s maximum limit of 250 pounds per square inch, the cylinders increased speed, until at full bore they were producing more than 2,400 horsepower. 

Enough to ultimately propel the train past 126 miles per hour (203 km/h), albeit for less than one minute on a lonely stretch of track just south of Grantham.  

The impressive feat and new world record surpassed the previous mark set by the German steamer BR 05022, which had hit 124 miles per hour (200 km/h) the previous year. 

Now more than 80 years later, the record set by the English steam locomotive still stands. 



Known collectively as A4 Pacifics, the attractive, powerful and revolutionary class of steam locomotives were the brainchild of Sir Herbert Nigel Gresley.

As Britain’s preeminent locomotive designer, Gresley ultimately rose to become Chief Mechanical Engineer at the London and North Eastern Railway (LNER).

However, despite the record breaking run by his state-of-the-art machine, Gresley remained ever humble, politely reminding everyone that his train had only been capable of maintaining that impressive speed for just a few short seconds. 

Data from the dynamometer did prove that a new record had been set, but some claimed that it shouldn’t have stood at all because at the time Gresley’s Mallard had been on a downward slope. 

In addition, its engine had been running at a speed at which it was never meant to operate, whereas the German train had been on level ground, purportedly with more umph left under the hood when it had set the previous record.   

On the other hand, the German machine had only weighed 197 tons (178,800 kg) compared to Mallard’s gross weight of 240 tons (217,700 kg). 

But though the original plan had called for the Mallard to carry on to London after breaking the record, a bearing in the lower end of the center cylinder became so hot that it partially melted, prompting an unscheduled stop at Peterborough for repairs. 

This issue highlighted a number of serious design and mechanical problems including poor cylinder alignment and inadequate tolerances between vital engine components, the result of which was that the center cylinder reciprocated more rapidly, did more work, and generated more heat than the ones on either side of it. 

These and other issues would be addressed and corrected on later models, but controversy, national pride and technical jargon aside, the Mallard’s record stood, and the event ushered in a new era of ultra-fast commercial passenger service between major cities in England and Scotland. 

However Gresley wasn’t one to rest on his laurels, and immediately afterward he and his seasoned team of engineers and drivers set out to up the ante yet again. 

They theorized that under the right conditions 130 mph (210 km/h) was possible, and that had Mallard not had to slow slightly while passing the rail junction at Essendine they may have hit it on the first attempt. 

With the outbreak of war in the fall of 1939 however, their brash plans took a backseat to far more pressing matters. 

Background & Design 

Featuring stunning art deco inspired styling, huge boilers and tiny cabin windows barely larger than a man’s face, at least in diehard railroad circles A4s were often referred to as 4-6-2-s for their unique wheel configurations pioneered by Gresley just a few years before. 

Under the Whyte running gear classification system, the numbers 4, 6, and 2 represent the position of the A4’s wheels – in this case four unpowered wheels on two axles up front, six powered wheels on three axles in the middle, and two trailing wheels on one axle in the rear. 

In photographs of A4s it appears as though there are eight more wheels – four on either side – in the rear of the unit behind the crew compartment. 

However, these were actually on the coal car or tender, which wasn’t technically part of the power unit itself, though considering their identical paint schemes and the narrow gap between them, it was difficult to tell where one stopped and the other started.  

Much of Gresley’s design inspiration came from a trip he’d taken to Germany in 1933, during which he’d been impressed by a new class of diesel locomotives known collectively as Flying Hamburgers – not for their resemblance to the popular food, but because they were fast and primarily operated in and around Hamburg. 

For a time the LNER actually considered purchasing German diesel trains for mainline use between London and Newcastle. 

After all, diesels were more efficient. 

But though they produced far less horsepower, they didn’t have multi-thousand gallon boilers prone to catastrophic explosions due to rust and stress fractures.

However, because the technology hadn’t been fully developed, reliability was a constant concern and the initial capital investment would have been prohibitively expensive. 

Thankfully for Gresley, these drawbacks worked in his favor. 

He knew that steam was still a viable alternative, and that with additional development he could produce locomotives that were even faster, more reliable and more efficient. 

As such, lessons learned from earlier A1 and A3s made them the perfect foundations on which to base the new A4s. 

Like A4s, in their time both A1s and A3s set a number of speed records themselves. 

Originally designed for main line passenger service with the Great Northern Railway, one such record was set by A3 “Papyrus” Number 2750 that hit 108 miles per hour (174 km/h) just a few years before. 


Another, the Silver Link, later hit 112 miles per hour (180 km/h) during a press event held to publicize its introduction into the fleet. 

Like their predecessors, A4 Pacifics were manufactured at LNER’s Doncaster Works, but though Gresley based his new design largely on his own previous locomotives, a number of elements were borrowed from the new breed of trains then being introduced in the United States, namely the K4s developed for the American Pennsylvania Railroad by the American Locomotive Company (ALCO).  

Incorporating a variety of new concepts and design features, Gresley’s A4s would operate under higher boiler pressure and have larger fireboxes and combustion chambers, and perhaps even more importantly, the consumption of both water and coal were significantly reduced making the trains even more economical. 

Another new feature that significantly improved efficiency was the Kylchap double-chimney, the first of which had been installed just before Mallard’s record run in March of 1938. 

Without this new double exhaust system it’s unlikely that the Mallard would have reached the speed it did, but ironically, cost-conscious company executives almost didn’t allow it to be installed in the first place because the cost of conversion was so high. 

In the end they relented, and after a brief break-in period it was determined that the exhaust system alone not only increased power, but reduced coal consumption by as much as 7 pounds (3.2 kg) per mile. 

Not surprisingly, many existing locomotives were retrofitted with Kylchap double-chimneys, and new units had them installed at the factory. 

Another area in which A4s outshined most of its competitors was aerodynamics. 

Featuring streamlined metal skirts or valances that covered nearly the entire train, only the wheels and running gear were exposed. 

In fact, A4s were among the first locomotives with bodies that acted as large aerofoils that cut through the air much like the fuselages of airplanes.  

On the downside, valances were expensive to install and maintain, made maintenance more difficult and decreased driver visibility. 

As a result, they were often removed, especially on trains that operated at lower speeds where aerodynamics weren’t so important. 



A4s entered service with the LNER to power the Silver Jubilee passenger trains that ran dedicated routes between London’s King’s Cross Railway Station and Newcastle.

Since most of the trains were manufactured in 1935 – the year of King George V’s 25-year Silver Jubilee – they were painted silver throughout and took the name to commemorate the historic event. 

Each train was originally about 460 feet (140 m) long and consisted of seven coach cars that together with the locomotive weighed approximately 380 tons (344,000 kg), though additional cars were added later that nearly doubled gross weight.

But though A4s were more than capable of sustained speeds in excess of 100 miles per hour (161 km/h), they typically operated much more slowly. 

No English steam or diesel locomotive before or since has matched the A4s’ top end performance, but despite operating primarily on the railroad’s East Coast Main Line which was especially suited to high-speeds, going that fast just wasn’t safe or economical. 

Higher speeds not only caused increased wear and tear on various components, but increased power output meant that overstressed engines burned through far more coal.

Average speeds generally hovered around 65 miles per hour (105 km/h), and the 285-mile   (458 km) runs between London and Newcastle typically took between 4 and 4.5 hours.  

Engine power was generally calculated mathematically using factors like speed, gross weight and grade. 

Under normal operating conditions, the A4s’ three-cylinder steam engines produced about 2,200 horsepower. 

At the time this was the highest output achieved by any English locomotive, though on some occasions when conditions were optimal they were capable of producing nearly 2,500 horsepower. 

This latter figure was calculated in the early ‘60s when the Mallard hauled 11 coach cars grossing nearly 400 tons (363,000 kg) up Stoke Bank at a sustained speed of nearly 80 miles per hour (129 km/h). 

Surprisingly, this colossal feat actually required more power and tractive effort than the world record run in 1938. 

By comparison, America’s Big Boy steam-powered locomotives cranked out nearly 6,000 horsepower and were capable of hauling loads in excess of 3,600 tons, though even on level ground they rarely exceeded 70 miles per hour (112 km/h). 

During the Battle of Britain much of England’s commercial rail service was temporarily suspended because slow moving trains make relatively easy targets, and because vital resources like coal and steel were in greater demand elsewhere.  

One A4 that was being stored in what’s now the National Railway Museum in York took a direct hit from a German bomb in late April of 1942, after which it was officially decommissioned and sold for scrap. 

Shortly after the cessation of hostilities A4s were once again pressed into unrestricted service, and in addition to providing power for the run between London and Newcastle, a new route to Edinburgh, Scotland was opened as well. 

The fastest official speed of the postwar era was recorded on May 23, 1959, when A4 number 6007 hit 112 miles per hour (180 km/h) on the downward slope of Stoke Bank.   

Of the 35 A4s built, the first four included “silver” in their names, two were named after English colonies and protectorates, most were named after birds, and the last, number 6007, was named after the man who’d designed them, Sir Nigel Gresley. 


The End of the Line 

Many Pacifics remained in service for more than three decades, after which they were steadily withdrawn in two stages ending in late 1966.

All were ultimately replaced by diesel locomotives that were more reliable, more fuel efficient and less expensive to operate. 

Also known as “streaks,” during their service lives A4s were popular subjects for painters, photographers and documentary film makers. 

Including A1 and A3 models, more than 100 Pacifics were built, two of which were renamed Dominion of Canada and Dwight D. Eisenhower after the Second World War to commemorate contributions made by Canada and the United States. 

Both were shipped to their new homes overseas after being withdrawn from service with British Railways, and of those that remained in England, a few were restored to near-original condition. 

As late as 2013, unit 4464 underwent a series of high-speed runs in homage to Mallard’s record breaking run in 1938. 

On that day Bittern topped out at 93 mph (149.7 km/h), and though she may have had more in her, level heads prevailed and she wasn’t pushed any farther. 

Between 2015 and 2020, Union of South Africa was the only operational A4 with an up-to-date mainline certificate, though she too was withdrawn in 2021 when significant boiler issues were discovered during routine maintenance. 

Though there are no longer any A4s fit for service, rumor has it that the Sir Nigel Gresley may once again be seen out on the rails after a complete overhaul that’s scheduled for completion in late 2022. 

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