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The AA20: The USSR’s “Big Boy”

Written by C. Christian Monson

During its existence, the Soviet Union was known to take things to the next level: big boats, huge rockets, giant bombs. One particular industry where they really went all out was the railroad. In addition to developing the 1,524-millimeter Russian broad gauge track that allowed for wider and taller trains than in the rest of Europe, they completed vast stretches of railway in the far east, such as the 896-mile (1,442-kilometer) Turkestan-Siberian Railway. In fact, during the Soviet era, freight rail traffic increased by 55 times and passenger rail traffic by 10 times. Before its collapse, the USSR boasted a railway network of almost 92,000 miles, around 147,000 kilometers, second only to the United States.

Perhaps the most ambitious example of the Soviets’ railroad obsession was the AA20 steam locomotive, also known as “Stalin’s Engine.” The only locomotive ever built with 14 driving axles, the AA20 stretched a whopping 110 feet, nearly 34 meters, making it the longest rigid-frame locomotive in the world until 1939, and the longest ever in Europe. 

Unfortunately, in this case, the Soviets may have been a bit too ambitious.



In the 1920s, the Soviet Union under Joseph Stalin sought to rapidly industrialize to catch up to Western powers like Germany, the UK and the US. Specifically, Stalin claimed the nation was “50 to 100 years behind the advanced countries,” and said he wanted to close that gap in just 10 years. 

A planned economy with the government directing the investment of capital and resources, the USSR developed their first “five-year plan” in 1928. The Soviet Union was vast to say the least. It was the largest country by area during its existence at over 8.5 million square miles, or 22 million square kilometers, almost five percent of the land on Earth. As a result, the nation’s resources were often far apart. 

For example, the Ural-Kuznetsk Metallurgical Combine needed coal and iron ore to produce steel, but the two resources were situated over 1,200 miles or 2,000 kilometers apart. Plus, the final steel product then had to reach its destinations, most of which were in the European part of the country where some 72% of the Soviet population lived.

The powers that were decided the best way to solve this was with freight rail. However, they didn’t want to spend too many resources or too much time developing the railway over other important industries. Therefore, the strategy involved limiting investment in the railroad so that it could just barely meet the demands of industrialization.

As you can probably guess, the trains regularly had problems coping with the volume of freight on the expanding lines. The railroad was in constant crisis with numerous failures of the supply line that resulted in production shut-downs. This was further complicated by the fact that there were four distinct groups trying to manage the railway: the Soviet government, the Communist Party, the Gosplan agency responsible for economic planning, and the railway ministry NKPS. Some wanted to invest in more trains while others wanted to completely replace all the tracks with electrified ones. 

A less expensive and faster option than building more trains or replacing the tracks, the Soviets finally decided to merely design more powerful locomotives that could pull more freight. This was easier said than done, though, because the tracks and freight cars could only handle so much weight and force.

The original Soviet locomotives were rigid steam engines of the 0-10-0 and 2-10-0 wheel configurations. Based on Whyte notation, this refers to the number of wheels the train has, ordered by non-powered leading wheels, powered drive wheels, and non-powered trailing wheels. In other words, the 2-10-0 had one axle of two leading wheels, five axles of 10 driving wheels that actually moved the train, and no trailing wheels.

Some Soviet engineers wanted to keep these wheelsets but merely increase the locomotive’s power and axle load, but this would have required reinforcing the tracks. Therefore, other experts proposed an alternative: adding more wheels. The United States and Germany had previously had success with steam engines as large as 4-12-2, so engineers looked to those designs. 

There was still an obstacle, though, which was that these foreign train engines had an axle load of 27 tonnes. The Soviet tracks could only handle 20, so if they wanted to avoid reinforcing the tracks, they’d have to add even more axles. Thus the plans for the 4-14-4 AA20 locomotive were born. 



The AA20 ran into hiccups from the get-go.

In 1931, a group of graduates from the Russian University of Transport developed the initial design which called for a 2-14-4 wheel configuration. Sponsored by Andrey Andreyev, the politician honored by the locomotive’s AA designation, the plans were sent to the Krupp steel works in Essen, Germany, with orders for two of the locomotives.  

However, it turned out that even a 2-14-4 configuration wouldn’t be able to handle the weight restrictions of the tracks, so the incomplete AA20 was transferred to the Voroshilovgrad Locomotive Plant in Lugansk, Ukraine, where it was to be finished in the 4-14-4 configuration. Unfortunately, the Lugansk plant was not very experienced at producing locomotives and was still in the process of mastering the smaller 2-10-2 that had also just been designed. As a result, construction went so slowly that the NKPS canceled the order for the second locomotive, settling for the single AA20 that was finished at the end of 1934.

In addition to being the longest steam locomotive at the time, the AA20 was the only one with seven driving axles in a rigid frame, a distinction it holds to this day. Not only that, but it had one of the largest steam boilers ever used on a European locomotive at 60 tonnes. This let the steam engine produce 7,200 horsepower, propelling it to a theoretical maximum speed of 156 miles an hour, around 250 kilometers per hour. 

Intended to haul heavy freight between Donbas and Moscow, the AA20 took its maiden voyage on 1 January 1935 during which it was supposed to provide publicity for the megaproject in the nation’s capital. However, its engineers quickly recognized a number of problems. For example, the steam distribution system didn’t function as designed leading to less power and speed than hoped.

More importantly, the train just wouldn’t stay on the tracks. It was too big. As a non-articulated locomotive, the AA20’s seven driving axles all remained parallel to one another with no ability to turn. With over five and a half feet (1.7 meters, to be exact) of space between each one, the locomotive had a rigid wheelbase of over 30 feet. It was too long to negotiate curves and derailed regularly.

Without a redesign, there was no way the AA20 could help alleviate the USSR’s railway crises.


Soviet railway officials tried a couple of ideas to get the AA20 to pass through curves without falling off the tracks.

The first was to remove the flanges from the wheels on the middle three driving axles. Flanges are the lips on the inside of train wheels that prevent them from moving laterally off the track. Removing them gave the locomotive a bit more flexibility on turns… but not enough.

The next idea was including universal joints in the coupling rods, the visible rods that connect each wheel and transfer power from the drive. Since a universal joint consists of a pair of hinges oriented at 90 degrees to each other, it allows rigid rods like a train’s coupling rods to transmit power while being oriented at inclined angles to one another. In other words, it would give the wheelbase the ability to bend slightly without losing power. These universal joints were installed between the first and last two axles.

These solutions failed to solve the problem, though. The AA20 kept derailing basically every time the engineers tried to run it. On top of that, it frequently spread the track and destroyed any “points” it passed over, the movable rails that can diverge a train from one set of tracks to another. Plus, it was also too long to fit on railway turntables and in the stalls at depots.

Worst of all, the AA20 proved incapable of actually hauling any freight because it was too powerful for the Soviet couplers in use at the time, which could only support a maximum 85 tonnes of coupling weight. It looked like solving the USSR’s railway problems would take more than just a big engine.



By 1941 and the USSR’s entry in World War II, Soviet railway freight had grown by 20 times to 400 billion tonnes per kilometer from just around 20 billion in 1928. Unfortunately, though, the AA20 had nothing to do with it. Instead, the expansion mostly utilized the FD, the steam locomotive with a 2-10-2 configuration that was produced in Lugansk at the same time.

In 1935, the Central Committee of the Communist Party of the Soviet Union appointed Lazar Kaganovich as minister of the railways to solve the railway crises. Although he spent much of his time busy purging the railway administration’s ranks of “saboteurs,” he also reformed the USSR’s freight transportation strategy by prioritizing bottleneck areas and investing in heavily trafficked lines while more or less abandoning other lines.

It was during this time that officials finally admitted that the AA20 was a fruitless endeavor. No amount of tweaking would make it work. They moved it into storage at the Shcherbinka test facility outside of Moscow where it was used for instructional purposes until it was finally scrapped without announcement or fanfare in 1960. 

In the end, the AA20 is considered by most experts to have been a total engineering failure. It was never even used in actual service. One thing, however, must be said. In their attempts to solve a complex problem, the Soviet planners definitely “went big.”


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