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.
|Class||Qty.||Road Number||Later Numbers||Year Built||Builder||Notes|
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.
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.
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 pulling a 20-car passenger train. See Locobase 338 for a description of the revised bearing design that permitted smooth riding at such speeds.
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.
When the UP ordered more 4-6-6-4s in 1944, they took the opportunity to substantially rework the boiler of the 1930s variants described in Locobase 1406. 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.
The tubes were shortened by 2 feet, 94 sq ft (8.7 sq m) of circulators were added to the firebox heating surface, the grate area jumped by 24 sq ft (2.2 sq m), superheater area increased by 91 sq ft (8.4 sq m), boiler pressure rose by 25 psi (1.72 bar), cylinder diameter shrank by an inch ....about all that didn't change was the driver diameter.
See Locobase 338 for a description of the revised bearing design that permitted smooth riding at high speeds.
Union Pacific's general mechanical engineer Arthur H. Fetters suggested basing a simple articulated design on the big 9000-series 4-12-2, thus invented 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
According to the locomotive diagram, the first 10 had Worthington feedwater heaters, then next 5 had Sellers had exhaust steam injectors, and the last 25 Worthingtons. This latter group arrived in 1937. They also put 5 more tons of adhesion weight, bulking up to 407,340 lb and increasing overall engine weight to 584,950 lb.
|Specifications by Steve Llanso|
|Railroad||Union Pacific (UP)||Union Pacific (UP)||Union Pacific (UP)|
|Road Numbers||3950-3969, 3975-3999||3930-3949||3900-3939 / 3800-3839|
|Locomotive Length and Weight|
|Ratio of driving wheelbase to overall engine wheebase||0.20||0.35||0.35|
|Overall Wheelbase (engine & tender)||106.67'||106.67'||97.87'|
|Axle Loading (Maximum Weight per Axle)|
|Weight on Drivers||403700 lbs||404200 lbs||399840 lbs|
|Engine Weight||627000 lbs||634500 lbs||566950 lbs|
|Tender Light Weight||436500 lbs||434500 lbs||322600 lbs|
|Total Engine and Tender Weight||1063500 lbs||1069000 lbs||889550 lbs|
|Tender Water Capacity||25000 gals||25000 gals||18106 gals|
|Tender Fuel Capacity (oil/coal)||28 tons||28 tons||6000 gals|
|Minimum weight of rail (calculated) on which locomotive could run||112 lb/yard||112 lb/yard||111 lb/yard|
|Geometry Relating to Tractive Effort|
|Boiler Pressure||280 psi||280 psi||255 psi|
|Cylinders (dia x stroke)||21" x 32" (4)||21" x 32" (4)||22" x 32" (4)|
|Tractive Effort||97352 lbs||97352 lbs||97305 lbs|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||4.15||4.15||4.11|
|Firebox Area||602 sq. ft||604 sq. ft||548 sq. ft|
|Grate Area||132 sq. ft||132 sq. ft||108.25 sq. ft|
|Evaporative Heating Surface||4817 sq. ft||4642 sq. ft||5304 sq. ft|
|Superheating Surface||2355 sq. ft||1741 sq. ft||1650 sq. ft|
|Combined Heating Surface||7172 sq. ft||6383 sq. ft||6954 sq. ft|
|Evaporative Heating Surface/Cylinder Volume||187.75||180.93||188.37|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||36960||36960||27604|
|Same as above plus superheater percentage||49157||46939||34229|
|Same as above but substitute firebox area for grate area||224185||214782||173278|