The Largest Steam Locomotive: The Union Pacific Big Boy

In the 1930s, the global political climate was heating up. With the wounds of World War One still fresh, and Adolf Hitler becoming Chancellor of Germany in 1933, tensions were rising between European powers; it was becoming clear a war was coming. Anticipating this, the United States’ incredible industrial might was beginning to speed up.

Union Pacific

Around this time in 1936, Union Pacific, a major US railroad company founded in 1862, were receiving deliveries of the Union Pacific Challenger steam locomotives. These large trains were designed for fast heavy freight hauling, and they could max out at 70 mph.

One area these trains operated was over the Wasatch mountain range between Ogden, Utah and Green River, Wyoming. This railroad was well known for its steep slopes. Slopes are a trains enemy. Pulling a load up a 1% hill gradient is twice as hard for the locomotive than on level ground. On this stretch of railroad, the gradient was up to 1.14%.

Five trains on a slope in the Wasatch mountains shows the effort required to move through this area.

Pulling trains through here was hard work for any engine, even for the huge Challengers. They required the assistance of ‘helper’ locomotives that provided the train with a boost, before disconnecting after cresting the hill. Sometimes up to three helpers were needed for a train to make it through this area.

These helper operations were costing Union Pacific time and money. With war looming, these were commodities that America couldn’t afford to waste. It was decided a new locomotive would be required, one with the brawn to take on this route alone.

The Union Pacific Big Boys

What they designed was a 5 metres tall, 40 metre long, 4-8-8-4 monster.

At almost half the length of a football pitch, they weighed 550 tonnes (1.2 million pounds) all in. 100 tonnes heavier than a fully loaded Boeing 747. A 300 psi boiler powered the massive 1.7 metre wide driving wheels, and had 135,000 lbs of tractive effort.

Due to their extreme length, the Big Boys had to be articulated. The 4-8-8-4 wheel layout meant 4 pivoting wheels at the front guided the engine, while 16 wheels provided power, and another 4 at the back supported the rear. The Big Boys were the only steam engines to use this setup.

The enormous Big Boy class. Image by Acorns Resort CC BY-SA 2.0

Originally, Union Pacific were going to name this class ‘Wasatch’, after their planned route, but after an unknown worker wrote ‘Big Boy’ on the front of the first model built, that was adopted as their name. A total of 25 were built between 1941 and 1944.

The Big Boys are not the largest steam engines ever built in weight, length and power, but no other locomotive beats them in all three categories. No matter what gauge is used, they rank among the top more than any other locomotive, and for that reason they are considered as the overall largest steam engines in history.

Especially when excluding failed, experimental and one off locomotives, the Big Boys are clearly the biggest successful design.

Workers next to a Big Boy’s running gear shows the scale of this locomotive. Image by David Brossard CC BY-SA 2.0

Their Performance

They were designed with speed and power in mind, comfortably reaching and cruising at 80 mph. With the size and power these locomotives provided, Union Pacific finally had an engine that could pull 3,200 tonnes of freight unassisted up the steep slopes of the Wasatch mountains.

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As Union Pacific gained experience with these powerhouses, Big Boys often hauled 4000 tonne trains up this route. On level ground, it could pull these loads at 60 mph.

No.4019 Big Boy pulling freight through Echo Canyon, Utah.

The Big Boys in Service

Arriving the same year Japan attacked the American naval base at Pearl Harbour, the Big Boys showed up just in time to use their muscle in aid of the ever increasing US war effort, where they proved invaluable.

A Big Boy photographed in 1941.

In their service, the total distance all Big Boys covered is around 25 million miles. They moved millions of tonnes of materials when the US needed it most. They continued after the war, where their power meant they would be some of the last steam trains in use, being retired in 1962.

17 of the 25 Big Boys would be scrapped over the coming decades, with the rest in museums. In 2013, Union Pacific pacific re-acquired No.4014 from the RailGiants Train Museum with plans to restore it back to running order. During its 20 year service it had covered 1,031,205 miles.

In May 2019, No.4014 moved under its own power for the first time in almost 60 years, and immediately became the largest operational steam locomotive on the planet. No.4014 is now part of Union Pacific’s historical fleet, rolling out for special events around the USA.

The restored No.4014 Big Boy. Image by Mobilus In Mobili CC BY-SA 2.0

With their size, power and weight, enthusiasts regard the Big Boys as the definitive steam locomotives and an icon of US industrial power.

Another One From Us: What is a Differential?

Usually, with such freaks of engineering, their impracticality often means they end up scrapped or a museum piece, but thanks to Union Pacific, the legendary Big Boy class can still be seen thundering along the railroad.