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

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--3975-3984-- 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.

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. (Thands to Chris Hohl for his 2 April 2023 email correcting the engine length and weight on the drivers and for including Alco builder's Order No S-1938.) 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 "Four-Cylinder Simple" 4664 S 566, 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 341 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. Alco's builder's card described the firebox as having a Gaines arch.

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).