In the 1930s, the Union Pacific was looking for a more powerful engine that could haul heavy passenger and freight trains faster over mountain lines like the Wasatch Grade. Although a twelve-coupled locomotive was already in use with the class 9000, the locomotive soon to be called the “Challenger” was designed as an articulated locomotive. While most other articulated locomotives were designed for high tractive effort at low speeds, the Union Pacific now designed these locomotives for speeds in excess of 60 mph or 97 km/h.

The first production batch, like all others, came from the ALCO plant in Schenectady and was designated class CSA-1. With an integrally cast frame and roller bearings on all axles, the targeted speeds could be easily reached. While the front, two-axle bogie ensured sufficient running stability, the trailing, two-axle bogie enabled a large firebox. 

The following year 25 CSA-2 were built, which were almost identical. One difference, however, was the approximately 10,000 pounds increased adhesive weight. Six of these locomotives were fitted with steam heating in order to be able to pull passenger trains. One difference within the first 40 engines was that some had a Worthington feed water heater and some Sellers exhaust injectors. 

After realizing that the Challenger could no longer handle the 3,600-ton freight trains on Wasatch Grade alone, an even more powerful locomotive was developed on its basis. This was the 4000 “Big Boy” class, which had two additional driving axles. Shortly thereafter, Otto Jabelman revised the Challenger, again incorporating innovations from the Big Boy. After the bigger sister received the class number 4884 based on its wheel arrangement, the new Challengers were now class 4664 instead of CSA.

The new batch now had a higher boiler pressure and smaller cylinders. A link between the chassis groups was installed, which improved the weight distribution. In addition, all locomotives were now equipped for use with passenger trains. The seven-axle tenders now held 56,000 pounds (25.4 t) of coal and 25,000 gallons (94,600 l) of water.

The first revised Challengers were the 20 class 4664-3 locomotives built in 1942. In 1943, 25 4664-4s followed, the only difference being that they were 6,500 pounds heavier. Since less high-strength steel was available due to the war, these had a heavier frame. Six identical locomotives went to the D&RGW, which went to the Clinchfield Railroad in 1946. The last 20 Challengers formed the 4664-5 class, delivered in 1944. In order to be able to classify these in the number range from 3900, the CSA-1 and -2 were renumbered as 3800 and the new locomotives were given the numbers 3930 to 3949.

Although all Challengers were delivered coal-fired, as early as 1937 the six CSA-2s equipped for passenger service were converted to oil firing. Instead of 22,000 pounds (20 t) of coal, the oil tenders now held 6,000 gallons (22,700 l) of oil and also 18,000 gallons (68,000 l) of water. All other CSA-1s and -2s were also converted to oil in 1942 and 1943. In 1950, ten of these were converted back to coal, and in the same year they were converted back to oil again. 18 class 4664-4 and -5 locomotives were also converted to run on oil. Some locomotives were given numbers starting at 3700 to differentiate between the types of fuel. The still coal-fired Challengers later received an extension on the tender, which increased the capacity from 28 to 32 short tons. 

Most Challengers were scrapped between 1956 and 1961. Two are still preserved today, both of which belong to the subclass 4664-4. The 3977 is not operational and is based in North Platte, Nebraska. The 3985 was back in service from 1981 and parked in 2010 due to technical problems. After Big Boy No. 4014 got operational, it was officially retired in 2020. In 2022, however, it was donated to the non-profit organization “Railroading Heritage of Midwest America” together with two other locomotives and is now being refurbished by them.

After the success of the Challenger, the Union Pacific planned to have only one locomotive haul express freight trains on Wasatch Grade and Sherman Hill in the future. To haul trains of 3,600 short tons and over it was calculated that an adhesive mass of 540,000 pounds would be required. The grate area should be large enough to be able to develop high power even with inferior coal. This required eight coupled axles and also four carrying axles in order to be able to carry a sufficiently strong boiler. At the same time, the locomotives should be designed for a speed of 80 mph. 

The locomotive developed in this way was the heaviest, but not the most powerful, conventional steam locomotive of all time. Like most modern American articulated locomotives, they had four cylinders with simple steam expansion and were therefore no true Mallets. The running numbers started from 4000, in fact the class was given the designation 4884 due to the lack of a name for the wheel arrangement. The Union Pacific first wanted to officially call it “Wasatch”, but the unofficial name “Big Boy” prevailed. In 1941, 20 class 4884-1 locomotives were built and in 1944 five more class 4884-2 locomotives.

In order to allow encounters between two Big Boys, the gap between the tracks had to be increased in many curves. The chassis had no problems with tight corners. In practice, they also pulled trains of 6,000 short tons and often reached their permitted speed of 70 mph with lighter trains. The number 4005 was converted to oil firing, but was converted back to coal firing after a short time. They later received an extension on the tender that increased capacity from 28 to 32 short tons. 

In the 1950s, the Big Boys were replaced by the gas turbine locomotives. The last commercial journey with a freight train took place in 1959. The 20 locomotives of the first series all achieved a mileage of just over a million miles. A total of eight Big Boys still exist today. The 4014 was the only one to be rebuilt and has been operational again since 2019. This makes it the most powerful operational steam locomotive today.

Since articulated freight locomotives in the 1920s did not yet reach the same speeds as ten-coupled locomotives, Union Pacific had the class 9000 developed with six coupled axles in one frame. The unique wheel arrangement with a four-wheel leading bogie, six sets of drivers and a two-wheel trailing truck was called “Union Pacific” by the UP. The 1926 prototype was officially given the class designation UP-1, while further batches were designated UP-2 to UP-5.

Only the fourth driving axle had no wheel flanges, but the first and sixth could be moved laterally by two inches. In order to be able to use the steam from the large boiler, three cylinders were installed. The middle one was offset upwards at an angle and was controlled by the outer cylinders via a Gresley valve gear.

In addition to the UP, two subsidiaries also received additional locomotives of this type, so that the total number ultimately reached 88. These were eight on the Oregon–Washington Railroad and Navigation and 15 on the Oregon Short Line Railroad. Although they were only intended for speeds of 35 mph (56 km/h), they often reached 45 to 50 mph with freight trains and could easily reach 60 mph (97 km/h) while running steadily.

Their area of operations focused on Nebraska, where they increased the speed of freight trains in the flat land. However, it became apparent that the Gresley valve gear's friction bearings were subject to high levels of wear. Only in eight of the oldest locomotives was this replaced with a Walschaerts valve gear and in the machines built from 1928 the Gresley valve gear was given roller bearings.

During the Second World War they had to work hard and were no longer adequately maintained. This meant that some were set aside during the war. The entire class was withdrawn from service between 1953 and 1956. The only surviving locomotive is the prototype number 9000. It is not operational and belongs to the Railway & Locomotive Historical Society in Pomona, California.

The 2-8-8-0 Mallet locomotives of the Union Pacific and its subsidiaries were known as classes MC-2 to MC-6. All batches were characterized by 57 inch drivers and cylinders with diameters of 26 and 41 inches and were built by ALCO-Brooks or ALCO-Schenectady. Their main area of operation were the grades in Wyoming, like the famous Sherman Hill. The UP called them “Bull Mooses”, a term that's sometimes used for this wheel arrangement in general.

Each of the five sub classes designates a batch with only minor changes compared to each other. The first one were 16 MC-2 built in 1918 by Schenectady, while all later batches were built by Brooks. These were 19 MC-3 in 1920 and ten MC-4 in 1923, the latter having somewhat differing dimensions in the boiler. The 20 MC-6 built in 1924 had fewer tubes, but a larger firebox heating surface thanks to thermic syphons.

Between 1936 and 1937, most were converted to burn oil, with their running numbers changed from the 3600 to the 3500 range. A bigger rebuild took place between 1937 and 1946, when they were simpled and the individual subclasses were renamed to SA-C-2 to SA-C-6. They got smaller cylinders with a diameter of only 23 inches and even the stroke was reduced. Even though the driver diameter was increased by two inches, the resulting tractive effort now was bigger than it had been previously in compound mode.

As early as in the late twenties, the introduction of the faster and more powerful class 9000 4-12-2 meant that the Bull Mooses were not used anymore for fast freights. When the Challengers were introduced, they were only used as helpers on the grades. In this role, they were retired between 1947 and 1954.

The only 4-8-2 Mountain locomotives ever operated by the Union Pacific were the classes MT-1 and MT-2, which were nearly identical apart from their firing method. The first 40 MT-1 were built in 1922 by ALCO, followed by 15 more in 1923 which weighed 3,000 pounds less. These were all coal-fired and had a tender that held 12,000 gallons of water and 20 tons of coal. Five MT-2 delivered in 1924 were oil-fired and had tenders with 15,000 gallons of water and 5,480 gallons of oil.

These locomotives were used to haul important passenger trains over long distances. They ran distances of 500 to 700 miles without major stops. Later, many were fitted with a partial casing. Only 7002 was totally streamlined and got roller bearings on all axles and the rods. It was used to haul the all-Pullman “Forty-Niner” when it was found out that early diesels could not keep up the schedule between Chicago and San Francisco. By 1956, all MT-1 and MT-2 locomotives had been scrapped.

When William Jeffer, President of the Union Pacific, got stuck on an incline with his train, the initial order for the FEF series came about. The class 7000 locomotive used at the time with a 4-8-2 wheel arrangement was to be given a more powerful successor for mixed service, which led to the 4-8-4 wheel arrangement. This wheel arrangement was already in use on other US railroad companies and led to the designation “Four-Eight-Four”.

The first 20 FEF-1 class locomotives were delivered by ALCO in 1938 and had 77 inch diameter coupled wheels. Rolling resistance was reduced with roller bearings on all axles and tapered connecting rods ensured that the moving masses were reduced. This allowed them to travel at speeds of 110 mph with ease and without compromising safety.

The following year, 15 FEF-2s were delivered, now with 80 inch coupling wheels, larger cylinders and higher boiler pressure. They were now cleared for 120 mph. Another ten FEF-3 followed in 1944, since no diesel locomotives were available for passenger trains due to the war.

The FEF series locomotives are known to produce between 4,000 and 5,000 hp at the drawbar and often traveled long distances in excess of 100 mph. In 1946 all were converted to oil firing, with the tenders now holding 6,550 gallons of oil instead of 25 short tons of coal.

In recent years, the locomotives were mainly used for freight trains before being retired at the end of the 1950s. Number 844 was the only US steam locomotive that was never officially retired and has since been used for excursion trips without major interruptions. In the meantime, it carried the number 8444 and is the only one of four surviving machines that is roadworthy.

In 1868 Rogers delivered five 4-4-0 locomotives to the Union Pacific, of which number 119 achieved great fame. When the gap on the First Transcontinental Railroad was closed on May 10, 1869, it was the locomotive that was brought by the Union Pacific, together with its Vice President, Thomas C. Durant, to the Promontory Summit in Utah. It took part in the ceremony there together with the Central Pacific's “Jupiter”, from which a famous photograph exists. In 1979 a replica was built by O'Connor Engineering Laboratories and it still exists today.

The first 16 of these oil-fired ten-wheelers were delivered to the Western Pacific in 1908, followed by 20 more the following year. ALCO-Brooks asked for a price of between 13,247 and 14,585 dollars per locomotive. These locomotives opened passenger service through the Feather River Canyon in 1910. They were soon retrofitted with superheaters. Most were retired in the thirties or fourties. The 94 was in service until 1953, featured at the Western Pacific 50th anniversary celebrations, and pulled excursion trains from 1979 to 1986.