Union Pacific 4-6-6-4 "Challenger" Type Locomotives

Class 4664-2/-3 (Locobase 341)

Firebox heating surface included 81 sq ft in circulators. These Challengers were revised from the earlier CSAs by Otto Jabelman with smaller pistons, larger grates, and higher boiler pressures. The table data is for class 4664-3 (3950-3969).

The 65 later engines were also 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.

6 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 1946 Union Pacific Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive collection.

When the UP resumed building 4-6-6-4s in 1944, they took the opportunity to substantially rework the boiler of the 1930s variant. The tubes were shortened by 2 feet, 94 sq ft of circulators were added to the firebox heating surface, the grate area jumped by 24 sq ft, superheater area increased by 91 sq ft, boiler pressure rose by 25 psi, 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.

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

Data from Union Pacific 11 - 1946 Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive collection. This group of 40 were first numbered 3900-3939, but later renumbered 3800-3839.

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
Class	4664-2/-3	4664-5	CSA-1/CSA-2
Locobase ID	341	6613	1406
Railroad	Union Pacific (UP)	Union Pacific (UP)	Union Pacific (UP)
Whyte	4-6-6-4	4-6-6-4	4-6-6-4
Road Numbers	3950-3999	3930-3949	3900-3939 / 3800-3839
Gauge	Std	Std	Std
Builder	Alco	Alco	Alco
Year	1942	1944	1936
Valve Gear	Walschaert	Walschaert	Walschaert
	Locomotive Length and Weight
Driver Wheelbase	12.17'	12.17'	12.17'
Engine Wheelbase	60.37'	35.08'	35.08'
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 per yard of rail on which locomotive could run	112 lb rail	112 lb rail	111 lb rail
	Geometry Relating to Tractive Effort
Driver Diameter	69"	69"	69"
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
	Heating Ability
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	4642	5304
Superheating Surface	2355	1741	1650
Combined Heating Surface	7172	6383	6954
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
Power L1	36424	29282	23843
Power MT	1193.48	958.27	788.79