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.
No. 3950 with a dynamometer car and test train at Sherman Hill
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.