The Union Pacific Challengers were a series of simple articulated 4-6-6-4 steam locomotives built by the American Locomotive Company (ALCO) between 1936 and 1944. Designed for both speed and heavy hauling, these locomotives served the Union Pacific Railroad until the late 1950s.
A total of 105 units were produced across five classes. Measuring nearly 122 feet (37 m) in length and weighing about 537 short tons (487 tonnes), Challengers were versatile workhorses that operated across much of the Union Pacific system. While primarily used for freight service, several were also assigned to passenger duties, notably hauling trains like the Portland Rose. Their performance and engineering directly influenced the development of the iconic Big Boy locomotives, whose design in turn refined the final three Challenger orders.
Today, only two Challengers remain. The most famous, Union Pacific No. 3985, was restored to operating condition in 1981 and served for decades in the railroad’s heritage excursion program. It was sidelined in 2010 due to mechanical issues, officially retired in 2020 after Big Boy No. 4014 returned to service, and later donated to the Railroading Heritage of Midwest America in 2022 for a second restoration. The other survivor, No. 3977, is preserved on static display in North Platte, Nebraska..
History[]
Description[]
The term Challenger refers to a class of articulated steam locomotives with a distinctive 4-6-6-4 wheel arrangement. This configuration consisted of four pilot wheels at the front to guide the locomotive smoothly into curves, two sets of six large driving wheels providing the main traction, and four trailing wheels supporting the rear of the locomotive and its massive firebox. Each set of drivers was powered by a pair of steam cylinders, effectively making the locomotive operate like two separate engines joined under a single, enormous boiler.
Union Pacific’s Challengers were built in five distinct classes, split into two main categories: the lighter CSA-1 and CSA-2 series, and the heavier 4664–3, 4664–4, and 4664–5 classes. They were conceived during a period when the railroad needed powerful, fast, and reliable locomotives capable of conquering the steep grades and long distances of its western routes—particularly the punishing climbs over mountain ranges.
Prior to the Challenger’s debut, most articulated locomotives were confined to slow-speed heavy freight work because of the limitations of their running gear. Union Pacific’s mechanical department, however, worked closely with the American Locomotive Company (ALCO) to break that mold. By incorporating high boiler pressures of 280 psi (1.93 MPa)—levels more typical of high-speed passenger locomotives such as the FEF series—and fitting 69-inch (1,800 mm) driving wheels, they created a locomotive capable of both raw pulling power and sustained speed. While most freight locomotives used smaller drivers for added torque, the Challenger’s larger wheels allowed them to comfortably cruise at over 60 mph (100 km/h), an almost unheard-of feat for articulated steam power at the time.
The first Challengers entered service in 1936 on Union Pacific’s main line over the Wasatch Range, between Green River, Wyoming, and Ogden, Utah. Despite their modern design, these early locomotives faced challenges of their own. The steep 1.14% eastbound climb out of Ogden was a serious obstacle—freight trains weighing 3,600 short tons (3,300 t) still required double-heading and helper locomotives. While the rest of the route featured gentler 0.82% grades, the Wasatch climb slowed operations considerably because adding and removing helpers was time-consuming. This operational bottleneck was one of the driving forces behind Union Pacific’s development of the even larger and more powerful 4-8-8-4 Big Boy locomotives in 1941.
When the Big Boy project was underway, Union Pacific’s chief mechanical engineer, Otto Jabelmann, took the lessons learned from that design and applied them to the Challenger. In 1941, the last three production orders of Challengers were significantly redesigned to improve performance and reliability. These “heavy” Challengers had a working weight of roughly 317 short tons (288 t), paired with a massive tender that weighed 174 short tons (158 t) when two-thirds loaded. Their calculated tractive effort reached an impressive 97,350 lbf (433.0 kN), making them formidable on long, fast freights. Under the revised operating plan, Big Boys were assigned to the steep Ogden climb, while the upgraded Challengers handled fast freight over the rest of the Wyoming main line with fewer delays.
Construction
A total of 105 Challengers were built for Union Pacific by ALCO between 1936 and 1944. Production came in five batches: the first two consisted of the lighter CSA-series locomotives, while the final three comprised the heavier, post-1941 redesigns. These orders coincided with a period of sharply rising freight demand, as the United States prepared for entry into World War II. During the war years, both Big Boys and Challengers worked tirelessly, moving record volumes of freight across the UP system. The Challengers proved to be among the most versatile large steam locomotives in American railroading—fast enough for passenger schedules, yet powerful enough for the heaviest freight assignments.
By the late 1950s, diesel locomotives replaced them in regular service, but the Challengers left behind a legacy of speed, strength, and mechanical innovation that directly influenced Union Pacific’s reputation for running some of the most advanced and capable steam locomotives ever built.
| Class | Quantity | Manufacturer | Serial Nos. | Year built | UP No. | Notes |
|---|---|---|---|---|---|---|
| CSA-1 | 15 | American Locomotive Company | 68745–68759 | 1936 | 3900–3914 | Converted to oil fuel in 1941–43; renumbered 3800–3814 in 1944. None preserved. All scrapped between 1957 and 1958. |
| CSA-2 | 25 | American Locomotive Company | 68924–68948 | 1937 | 3915–3939 | Converted to oil fuel; renumbered 3815–3839 in 1944. None preserved. All scrapped in 1958. |
| 4664-3 | 20 | American Locomotive Company | 69760–69779 | 1942 | 3950–3969 | 3968 converted to oil fuel in 1946, renumbered 3944 in 1946. None preserved. All scrapped between 1958 and 1959. |
| 4664-4 | 31 | American Locomotive Company | 70158–70162
70169–70182 70678–70683 |
1943 | 3975–3999 | 31 built but only 25 delivered to UP (see below); 3975–3984 converted to oil fuel in 1945; renumbered 3708–3717 in 1952. No. 3985 in excursion service from 1981 to 2010. 3977 preserved and on static display. Remainder scrapped in 1957. |
| 4664-5 | 20 | American Locomotive Company | 72792–72811 | 1944 | 3930–3949 | 3930/31/32/34/37/38/43/44 converted to oil fuel in 1952 and renumbered 3700–3707. None preserved. All scrapped between 1957 and 1959. |
As part of Union Pacific’s fourth Challenger order in 1943, the American Locomotive Company (ALCO) constructed thirty-one heavy 4-6-6-4 locomotives to the same advanced specifications developed under Otto Jabelmann’s redesign. However, wartime production priorities brought an unexpected twist to the order. The War Production Board, working through the War Department’s Defense Plant Corporation, redirected six of these newly built engines to the Denver and Rio Grande Western Railroad under a wartime lease agreement.
These six locomotives—numbered 3900 through 3905 on the Rio Grande—were classified as the railroad’s Class L-97. They proved highly capable on the mountain grades of the Rio Grande system, handling both freight and occasional passenger work during the demanding years of World War II. After the war, in 1947, the Rio Grande sold the engines to the Clinchfield Railroad, where they were renumbered 670 through 675 and reclassified as Class E-3. On the Clinchfield, they continued to demonstrate their speed and pulling power, but as diesel locomotives began to dominate, the Challengers were gradually phased out. By 1953, all six had been retired from active service.
Preservation[]
Out of the original 105 Union Pacific Challengers, only two locomotives have survived into preservation, both from the 4664-4 class built in 1943. Union Pacific No. 3977: This locomotive is preserved as a static display in Cody Park, North Platte, Nebraska, serving as a public monument to Union Pacific’s steam heritage.
Union Pacific No. 3985: Perhaps the most famous surviving Challenger, No. 3985 was returned to operating condition in 1981 by Union Pacific’s Steam Program in Cheyenne, Wyoming. It became a staple of the railroad’s excursion fleet for nearly three decades, hauling public and special trains across the Union Pacific system. In October 2010, mechanical issues sidelined the locomotive, and it was placed back into storage. Officially retired in January 2020, No. 3985 was later donated to the Railroading Heritage of Midwest America (RRHMA). It was moved to the organization’s Silvis, Illinois, shops—formerly the Rock Island’s Little Silvis Shops—where it is now undergoing an extensive restoration with the goal of returning it to operational service once again.
