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.
Class 4664-3/4 (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). (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 http://www.steamlocomotive.com/challenger/renumbering.php
for the complicated details.
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.
Class 4664-5 (Locobase 6613)
Data from 1946 Union Pacific Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive Raildata collection.
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.
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 Raildata 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.