Union Pacific 4-6-6-4 "Challenger" Locomotives in the USA

The Union Pacific Railroad took delivery of the very first locomotive with the 4-6-6-4 wheel arrangement in 1936 when it received 15 of them from the American Locomotive Company. These newly named "Challengers" were designated Class CSA-1.

In 1937, another 25 ALCO-built "Challengers" were added to the roster. This group, designated Class CSA-2, was given road numbers 3915 through 3939. They were similar to the Class CSA-1s. Six of them, numbers 3934 through 3939, were equipped for passenger service.

In 1942, ALCO delivered 20 Class 4664-3 "Challengers" which were numbered 3950 through 3969. The tenders on these locomotives were larger than either of the CSA classes.

In 1943, another 25 Class 4664-4 "Challengers" came from ALCO and were numbered 3975 through 3999. This group was very similar to the Class 4664-3s except that each weighed 6,500 pounds more.

A final 20 ALCO-built "Challengers" arrived in 1944 giving the Union Pacific a total of 105 of the 4-6-6-4s. These locomotives designated Class 4664-5 were similar to the Class 4664-3s except for an additional 7,500 pounds in the total weight. They were numbered 3930 through 3949 which required that the Class CSA-1 and CSA-2 locomotives be renumbered into the 3800 series.

An article by King in "Trains" in the early 2000s stated that the later UP Challengers cost around $130,000.00.

There are two surviving Union Pacific Class 4664-4 "Challengers", number 3977 at Cody Park in North Platte, NE and number 3985 which is operational and used in excursion service by the Union Pacific.


ClassQty.Road NumberLater NumbersYear Built Builder Notes
CSA-2 63934-39393834-38391937ALCO3

Please see this page for more details on UP Challenger renumbering, coal versus oil burners, paint schemes, and other details on the various classes of UP Challengers.

  1. Delivered with road numbers 3900-3914 and renumbered 3800-3814 in 1944. Converted to oil burners in 1943. Numbers 3801, 3802, 3804, 3807, 3809, 3811 & 3814 were converted to burn coal in 1950 and renumbered in the 3700 series and later that same year converted back to burn oil and retuned to the 3800 series numbers. Numbers 3800-3814 scrapped between 1956 and 1961.
  2. Delivered with road numbers 3915-3933 and renumbered 3815-3833 in 1944. Number 3915 converted to burn oil in 1942. Numbers 3817 & 3818 converted to burn coal in 1950 and renumbered in the 3700 series and later that same year converted back to burn oil and returned to the 3800 series numbers. Numbers 3815-3833 scrapped between 1956 and 1960.
  3. Delivered with road numbers 3934-3939 and renumbered 3834-3839 in 1944. Numbers 3934-3939 delivered equipped for passenger service and converted to burn oil in 1937. Numbers 3934-3939 scrapped between 1956 and 1960.
  4. Numbers 3950-3969 scrapped between 1956 and 1961.
  5. Numbers 3975-3984 converted to burn oil and renumbered 3708-3717 in 1952. Numbers 3975-3999 scrapped between 1959 and 1961.
  6. Numbers 3930-3932, 3934, 3937, 3938, 3943 & 3944 were converted to burn oil and renumbered 3700-3707 in 1952. All scrapped between 1952 and 1960.

Class Details by Steve Llanso of Sweat House Media

Class 4664-3/4 (Locobase 341)

The table data is for class 4664-3 (3950-3969). (Thanks to Chris Hohl for querying the class ID and road numbers for this class.) See Raymond Gutierrez's renumbering of the UP 4-6-6-4s at Wes Barris's [] for the complicated details. Works numbers were 69760-69779 in May 1942; 70158-70162, 70169-70182 in June 1943; and 70678-70683 in July.

Firebox heating surface included 83 sq ft (7.71 sq m) in circulators. These Challengers were revised from the earlier CSAs by Otto Jabelman with smaller cylinders, larger grates, and higher boiler pressures. In fact, the design shows an intent to wring the hottest steam out of the boiler and cram it into the smallest possible cylinder volume. Cutting 2 ft (610 mm) off the tubes and flues established a nearly perfect 107:1 length/diameter ratio. The high pressure setting and high superheat ratio allowed smaller-diameter (12"/305 mm) piston valves on all four cylinders.

Another benefit of the smaller power dimensions was likely a reduction in forces on the rails, especially when combined with running gear improvements. These included roller bearings on all axles (Timken on the drivers, SKF on the trucks). In addtion, the -3 and -4 designs were fitted with a vertical hinge between the driving units so positioned as to even the weight distribution between the front and rear driver sets. These engines could run at 70 mph (113 kph) pulling a 20-car passenger train. See Locobase 338 for a description of the revised bearing design that permitted smooth riding at such speeds.

The 60 later engines in the UP's run of 16-axle articulateds were not designated by the UP as Challengers, but simply 4664-3 through 4664-5. The 4664-3s were numbered 3950-3969 from the start and never changed (except for 3968, which was converted to oil-burning in 1946 and renumbered 3944.

The 4664-4s were delivered as coal burners and numbered 3975-3999. The first ten were renumbered 3815-3824 in 1944, regained their original numbers in 1945 when they were converted to oil-burning, were coal-burners in 1946, converted again to oil-burning in 1949, and renumbered 3708-3717 in 1952. They were scrapped in 1959-1960 with their original numbers assigned.

The 3975-3999 remained coal burners throughout their careers. 3985-3993 were renumbered 3825-3833 in 1944, but took back their original numbers later, probably in 1946.

Six more locomotives of identical design were leased as #3800-3805 for the Denver and Rio Grande Western in 1943, which returned them in 1946. At this point they went to the Clinchfield as E-3 (670-675) and retired in 1953.

Class 4664-5 (Locobase 6613)

Data from UP 11 - 1946 Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. Works numbers were 72792-72811 in August 1944.

When the UP ordered more 4-6-6-4s in 1944, Alco used the power dimensions, larger grate and higher boiler pressure of the 1942 design shown in Locobase 341, but reverted to a Schmidt Type A superheater. The builder changed the balance between 2 1/4" flues and 5 1/2" flues by deleting 46 tubes in favor of 8 more flues. Firebox heating surface area included a combustion chamber and 94 sq ft (8.7 sq m) of circulators.

Wartime limits on high-strength steels for locomotives meant heavier frames and the highest engine weights among UP Challengers. Hoping to confuse future compilers of steam locomotive information, the railroad renumbered the last ten locomotives of its earlier CSA-2 class and reused the original 3930-3939 for the first ten of this 1944 class.

See Locobase 338 for a description of the revised bearing design that permitted smooth riding at high speeds.

Eight of the class--3930-3932, 3934, 3937-3938, 3943-3944--replaced grates with oil burners in 1952. At that point, they were renumbered 3700-3707.

Class CSA-1/CSA-2 (Locobase 1406)

Data from Union Pacific 10-1936 and 11 - 1946 Locomotive Diagram books supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. (Thanks to Chris Hohl for his 10 December 2016 email supplying the builder's card for this class and for noting the original tender capacities.) This group of 40 were first numbered 3900-3939, but later renumbered 3800-3839. Works numbers were 68745-68759 in September 1936, 68924-68948 in August 1937.

Union Pacific's general mechanical engineer Arthur H. Fetters suggested basing a simple articulated design on the big 9000-series 4-12-2, thus inventing the Challenger wheel arrangement. See Locobase 338 for a description of the revised bearing design adopted in 1940 that permitted smooth riding at higher speeds. But Alfred Bruce's summary of Challenger capabilities can be applied to all 4-6-6-4s. Locomotives with this wheel arrangement were, he said, "...of a size that could be handled without too much difficulty, was well balanced and accessible, and had adequate ashpan capacity. In addition, it had all modern improvements, including integral cast-steel bed frames and roller bearings on all axles." (The Steam Locomotive in America, 1952, p. 328.)

UP eventually bought 105 4-6-6-4 locomotives for express passenger and freight service. See Locobases 331 and 6613

All four cylinders used 12" (305 mm) piston valves to admit steam. When coal fired, 19-C-1 tenders behind the first fifteen held 22 tons of coal fed through a BK stoker, weighed 116,600 lb (52,889 kg) empty and 310,000 lb (140,614 kg) loaded. Their six axles turned in SKF roller bearings. The latter 25 engines arrived with 19-C-2 tenders with the same capacities but weighing about a ton more empty and full.

The first ten had Worthington 6SA feedwater heaters, then next five had Sellers exhaust steam injectors, and the last 25 Worthingtons. This latter group arrived in 1937. They also put five more tons of adhesion weight, bulking up to 407,340 lb (184,767 kg) and increasing overall engine weight to 584,950 lb (265,329 kg).

Principal Dimensions by Steve Llanso of Sweat House Media
Locobase ID341 6613 1406
RailroadUnion Pacific (UP)Union Pacific (UP)Union Pacific (UP)
Number in Class452040
Road Numbers3950-3969, 3975-39993930-3949/3700-37173900-3939/3800-3839
Number Built452040
Valve GearWalschaertWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)12.17 / 3.7112.17 / 3.7112.17 / 3.71
Engine Wheelbase (ft / m)60.37 / 18.4035.08 / 10.6935.08 / 10.69
Ratio of driving wheelbase to overall engine wheebase 0.20 0.35 0.35
Overall Wheelbase (engine & tender) (ft / m)106.67 / 32.51106.67 / 32.5197.87 / 29.83
Axle Loading (Maximum Weight per Axle) (lbs / kg)
Weight on Drivers (lbs / kg)403,700 / 183,115404,200 / 183,342399,840 / 181,365
Engine Weight (lbs / kg)627,000 / 284,403634,500 / 287,805566,950 / 257,165
Tender Loaded Weight (lbs / kg)436,500 / 197,993434,500 / 197,086322,600 / 146,329
Total Engine and Tender Weight (lbs / kg)1,063,500 / 482,3961,069,000 / 484,891889,550 / 403,494
Tender Water Capacity (gals / ML)25,000 / 94.7025,000 / 94.7018,106 / 68.58
Tender Fuel Capacity (oil/coal) (gals/tons / ML/MT)28 / 25.5028 / 25.506000 / 22.70
Minimum weight of rail (calculated) (lb/yd / kg/m)112 / 56112 / 56111 / 55.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)69 / 175369 / 175369 / 1753
Boiler Pressure (psi / kPa)280 / 19.30280 / 19.30255 / 17.60
High Pressure Cylinders (dia x stroke) (in / mm)21" x 32" / 533x813 (4)21" x 32" / 533x813 (4)22" x 32" / 559x813 (4)
Tractive Effort (lbs / kg)97,352 / 44158.1897,352 / 44158.1897,305 / 44136.86
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.15 4.15 4.11
Heating Ability
Firebox Area (sq ft / m2)602 / 55.93604 / 56.13548 / 50.93
Grate Area (sq ft / m2)132 / 12.26132 / 12.27108.25 / 10.06
Evaporative Heating Surface (sq ft / m2)4817 / 447.514642 / 431.415304 / 492.94
Superheating Surface (sq ft / m2)2355 / 218.781741 / 161.801650 / 153.35
Combined Heating Surface (sq ft / m2)7172 / 666.296383 / 593.216954 / 646.29
Evaporative Heating Surface/Cylinder Volume187.75180.93188.37
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation36,96036,96027,604
Same as above plus superheater percentage49,15746,93934,229
Same as above but substitute firebox area for grate area224,185214,782173,278
Power L136,42429,28223,843
Power MT1193.48958.27788.79

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