The reference for locomotives and railcars
Union Pacific class 3900 “Challenger”
classes CSA and 4664
United States | 1936 | 105 produced
The last Challenger built, no. 3949
The last Challenger built, no. 3949
collection Rich Driver

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
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.

VariantCSA-1, CSA-24664-3 and 44664-5
Axle config4-6-6-4 (Challenger (Mallet)) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Length121 ft 10 7/8 in
Wheelbase59 ft 11 in60 ft 4 1/2 in
Fixed wheelbase12 ft 2 in
Total wheelbase98 ft 10 1/2 in106 ft 8 in
Service weight566,950 lbs627,000 lbs634,500 lbs
Adhesive weight399,840 lbs403,700 lbs404,200 lbs
Total weight876,950 lbs1,063,500 lbs1,069,000 lbs
Axle load66,700 lbs67,300 lbs67,400 lbs
Water capacity18,000 us gal25,000 us gal
Fuel capacity44,000 lbs (coal)56,000 lbs (coal)
Power sourcesteam
Estimated power5,000 hp (3,729 kW)5,800 hp (4,325 kW)
Optimal speed33 mph38 mph
Top speed70 mph
Starting effort97,305 lbf97,352 lbf
Power Plant
Driver diameter69 in
Boiler pressure255 psi280 psi
Expansion typesimple
Cylindersfour, 22 x 32 infour, 21 x 32 in
Grate area108.3 sq ft132 sq ft
Firebox area548 sq ft602 sq ft604 sq ft
Tube heating area4,756 sq ft4,215 sq ft4,038 sq ft
Evaporative heating area5,304 sq ft4,817 sq ft4,642 sq ft
Superheater area1,650 sq ft2,355 sq ft1,741 sq ft
Total heating area6,954 sq ft7,172 sq ft6,383 sq ft
Calculated Values
steam locomotive
Arthur H. Fetters
Otto Jabelman
last changed: 06/2023

We use cookies to save the following settings:

  • selected navigation structure
  • selected language
  • preferred units
  • spelling of railway company names

If you refuse the use of cookies, the settings will only be retained for the current session and will be reset to the default values the next time you visit the site.

Display of units

Here you can set the desired unit system for the technical data.

  • Metric: Lengths in meters, weights in tonnes, and volumes in cubic meters
  • Imperial (UK): Lengths in feet/inches, weights in long tons and volumes in imperial gallons
  • Imperial (US): Lengths in feet/inches, weights in pounds, and volumes in US gallons
  • Individual: Depends on the country of origin of each locomotive
Operator names

Here you can set the display of railway company names.

  • Short: Abbreviation or short form of the name
  • Standard: commonly used name, partially translated to English
  • Complete: full name in local language