What Is the Largest Locomotive Ever Built?

The mighty locomotive has played a key role in global economic growth during the past several centuries. While modern locomotives rely more on electrical power as their main energy source, they would not be around were it not for their steam and diesel predecessors and some of these classic engines rank among the largest locomotives ever built.

The largest steam locomotive ever built is Union Pacific’s 4000 series (aka the “Big Boys”), one of which, the 4014, is still operational today. Union Pacific also claims the largest diesel locomotive ever built, known as the Centennial, and named for the railroad’s 100-year celebration in 1969.

Locomotives have been around for hundreds of years, and even today, they play an indispensable role in commerce and transportation. Not long ago, only the largest locomotives could be trusted with the toughest jobs, and even by modern standards, steam and diesel engines have not only stood the test of time, but they have retained their title as the largest locomotives ever built, even to this day.

What Is the Largest Locomotive Ever Built?

While today’s locomotives feature advanced technologies that make them far more powerful and efficient than the world has ever seen, there was a time not too long ago when their performance was directly tied to their size. Simply put, the larger the locomotive, the more horsepower and tractive force (i.e., the amount of pull) it could be expected to produce.

This is why some of the largest locomotives ever built are of the steam and diesel varieties, as both engine types dominated the locomotive landscape before electric versions came along. Even compared to today’s electric locomotives, steamers and diesels more than hold their own.

Steam and diesel locomotives have played such vital roles in the growth and development of the railroad industry and are so embedded in the overall history of rail transportation that they are each worthy of individual attention in their own right.

Here is a closer look at history’s largest locomotives, starting with the one aptly named the “Big Boy”.

Geoffrey Fandl photo

The Largest Steam Locomotive Ever Built – The “Big Boy”

Throughout the early to mid-1900s, steam locomotives dominated rail networks throughout the US and in many parts of the world. The development of the steam engine contributed to rapid economic growth by allowing large loads of freight to be transported over great distances and across inhospitable terrain. For these and many other reasons, steam locomotives were synonymous with rail transportation.

Union Pacific’s 4000 series locomotives, affectionately known as the “Big Boys”, are widely considered to be the largest steam locomotives ever built. Only 25 of these locomotives were manufactured by the American Locomotive Company (ALCO) and Union Pacific began taking deliveries of them in 1941 against the backdrop of World War II.

The sheer size and scale of the Big Boys and their powerful attributes are impressive, even by today’s modern standards:

  • Its overall length (consisting of the engine and the tender) is 132 feet, 9 7/8 inches
  • Each Big Boy weighs 1,189,500 pounds
  • Each locomotive could carry 56,000 pounds of coal to fuel the steam engine along with 25,000 gallons of water
  • Each Big Boy could produce 135,375 pounds of tractive force
  • Being a steamer engine, this locomotive was capable of generating 300 PSI of steam pressure
  • As far as power, the Big Boys could produce up to 6,290 horsepower
  • Despite its mammoth size, this steam locomotive could reach a top speed of 70 miles per hour

The Big Boys were purpose-built to work freight rail lines linking Odgen, Utah, and Cheyenne, Wyoming. Their size and power enabled them to transport heavy loads through the rugged Wasatch Mountains, where steep slopes and winding passes presented challenging conditions. But thanks to their might and agility, the Big Boys (they were supposed to be named the “Wasatch” class) were up to the task.

union pacific bigboy
Jonathan Lee

The Triumphant Return of the Big Boy

The Big Boys were commissioned exclusively for Union Pacific and of the original 25 that were built, only eight remained after they were retired from service roughly 60 years ago (the rest were scrapped for their valuable resources) and these were placed into railroad museums and put on permanent display. Until that is, an ambitious plan was hatched to put one of them back into operation for a very special occasion.

In 2021, Union Pacific Big Boy 4014, newly restored and modernized, took to the rails once again to commemorate the 150th anniversary of the historic driving of the golden spike, an event signifying the official joining of the Transcontinental Railroad linking the eastern and western United States. The restoration process was a major undertaking given the Big Boy’s immense size:

  • The major components of the locomotive had to be disassembled, including the 250-ton boiler (which had to be hoisted off of the frame with the aid of two cranes)
  • The rear engine, weighing 14,000 pounds, also had to be separated from the frame
  • Each of the four main wheel sets, weighing 16,000 pounds each, had to be removed
  • The locomotive’s main manifold, individual headers, and complex network of feed pipes, all had to be dismantled and individually restored
  • Re-assembling the locomotive’s massive parts (even the articulation joint weighs one ton) required two heavy side boom cranes and a heavy steel sling

When all was said and done, Union Pacific’s  Big Boy 4014 underwent a complete restoration process that left no part untouched.

Perhaps most significantly, the meticulous refurbishment process not only restored Big Boy 4014 to its former glory but by once again rumbling down the tracks for its commemorative tour through 10 states, this historic engine added to its lore by not only reasserting its claim to being the largest steam locomotive ever built but also being an operational one (roughly 60 years after it was retired) to boot.

up 4014
Jonathan Lee

The Transition From Classic Steamers to Modern Diesels

For over a hundred years, steam locomotives dominated the rails and played a significant role in ushering in the industrial era that saw economies around the world sprout and prosper. While its reign as ruler of the tracks was a fairly lengthy one, advancements in engine technology led to the development of a new type of train engine that came to be known as the diesel locomotive.

Relying on internal combustion engines to generate electricity and providing power to axles that drive the wheels, the diesel locomotive (technically, diesel-electric) was warmly embraced by railroad companies who were finding steam locomotives too expensive to fuel and maintain.

With their greater efficiency came significant cost savings and before too long, diesels began to replace steam locomotives after World War II came to an end. It was during this period that the torch was passed from steamers to diesel locomotives.

From the 1950s and on, diesel locomotives were the standard engine type in rail transportation, and once again, Union Pacific would lay claim to having the largest diesels ever built with its iconic 6000 series Centennial locomotives.

The Largest Diesel Locomotive Ever Built – The “Centennial”

Union Pacific’s Centennial DD40X is not only the largest diesel locomotive ever built but also the most powerful. In all, there were 47 Centennials built exclusively for Union Pacific between 1969 and 1971 and there are 14 still in existence today (on permanent display in rail museums and public spaces).

Because they are based on combustion engine technology and are far more efficient than their steam predecessors, diesel locomotives tend to be smaller in size than steam locomotives. But this is all relative and diesels are by no means diminutive in nature; on the contrary, they are massive vehicles and Union Pacific’s Centennials are the largest of them all:

  • Each Centennial measures 98 ½ feet in length
  • A single locomotive actually consists of two 16-cylinder diesel engines, two onboard generators, and 8 traction motors, all on the same frame
  • The total weight is 545,432 pounds
  • This diesel is capable of reaching a top speed of 90 miles per hour

Incredibly, one of Union Pacific’s Centennials, Number 6936 (all of these diesels were numbered in the 6900s to commemorate the 100th anniversary of the driving of the Golden Spike in 1969) is still operational to this day, running special-event routes and promotional tours from its home base in Cheyenne, Wyoming.

There are plans, however, to formally retire this last remaining Centennial (at least from Union Pacific service) and donate it to the Railroading Heritage of Midwest America where it will continue in a prominent role of public display and interaction.

For a period of 15 years, Union Pacific’s fleet of Centennial diesel locomotives was entrusted with transporting all manner of freight throughout the railroad’s extensive rail network, and from 1969 to 1984, they represented the gold standard in diesels. To this day, the Centennials are remembered not only as reliable workhorses that dutifully performed their tasks but also as the largest diesel-electric locomotives ever built.

dda40x locomotive
H. Michael Miley

How Do Steam and Diesel Measure Up to Today’s Locomotives?

As impressive as the largest steamers and diesels are in terms of size and power, they pale in comparison to the scale and performance of modern locomotives. More specifically, the electric locomotives of the modern era are true behemoths capable of producing levels of tractive force and raw power that would have been unthinkable back in the heyday of steam and diesel engines.

Recently, the Shen 24, a collaboration between China’s Baoshen Railway Group and several government agencies, rolled off the assembly line and took to the rails. In so doing, the Shen 24 became the world’s most powerful electric locomotive and placed itself among elite company with staggering characteristics like these:

  • At a touch over 347 feet (106 meters) long, the Shen 24 is over 200 feet longer than Union Pacific’s Big Boy steam locomotive
  • The Shen 24 comprises 6 carriages and 24 axles
  • This electric locomotive can generate 28,800 kW and achieve a tractive force of 2,280 kN, the equivalent of 512,544 pound-force (lbf)
  • The Shen 24 can achieve a top-end speed of roughly 75 miles per hour (120 kilometers per hour)
  • More impressively, this electric locomotive has the power to pull a 10,000-ton train over a slope of 1.2 percent

The Shen 24’s physical attributes only tell half of its impressive story. It also features an array of cutting-edge smart technologies that makes it easier and safer to operate through features like automatic start and stop, self-adjusting speed control, and intuitive braking. This electric locomotive is more than a glimpse into the future of locomotives, it is an eye-opening demonstration of the here and now.

intermodal trains
Bill Hooper

Are Electric Locomotives True Locomotives?

But despite the Shen 24’s impressive traits and features, as far as laying claim to the title of largest locomotive ever built, there is an argument to be made that the Big Boy steamer and Centennial diesel are not in any jeopardy of losing their crowns. In fact, according to some rail transportation purists, the Shen 24 may not even be a true locomotive.

By definition, a locomotive should be a self-propelled vehicle, meaning that it can produce the motion necessary to move itself (and any train cars behind it). Put another way, a locomotive is presumed to comprise an onboard engine that generates the power needed to result in motion. To illustrate:

  • Steam locomotives produce motion by burning fuel like coal, wood, or oil to heat water in a large boiler to create steam which in turn powers a steam engine that drives the wheels
  • Diesel locomotives feature internal combustion engines that drive generators that create electricity which is then fed to electric motors that drive the axles to produce motion

In contrast, electric locomotives like the Shen 24 and countless others like it, do not have onboard engines nor are they capable of generating their own power. Instead, this type of locomotive receives electricity from external sources like overhead pickups or electrified third rails to power electric motors on the wheels or axles.

To put it another way, without the electricity supplied to it via electrical hookups, an electric locomotive simply could not move. As such, they do not satisfy the element of being capable of self-propulsion and in the eyes of some, are therefore not true locomotives like their steam and diesel counterparts.

Final Thoughts

Few inventions have had a far-reaching impact on commerce and transportation like the locomotive. Advancements in technology have made modern locomotives more efficient and powerful than their predecessors. But in terms of size and prowess on the rails, steam and diesel engines are the largest locomotives ever built and they have forever cemented their place in rail history.


Lifelong Rail Enthusiast and Owner of Worldwide Rails

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