In the Whyte notation for classifying steam locomotives by wheel arrangement, a 4-6-6-4 is a railroad steam locomotive that has four leading wheels followed by two sets of six coupled driving wheels and four trailing wheels. 4-6-6-4's are commonly known as Challengers.

The 4-6-6-4 represented the zenith of steam locomotive innovation. It was equipped with state-of-the-art features including roller bearings on all axles—both engine and tender—a massive …

2023年2月1日 · The 4-6-6-4 Challenger was the most successful articulated steam locomotive design. Union Pacific Challenger 3953 rounds a curve near Sherman, Wyo., highest point on the Overland Route at 8,013 feet, with an eastbound freight in 1951.

2006年6月13日 · They hauled mostly freight, although in the late 1930s, Challengers were used to power the railroad’s crack passenger train, the North Coast Limited, between Missoula and Livingston, Mont. The Northern Pacific went on to order 35 more 4-6-6-4s in three different classes-the last steam engines bought by the railroad.

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.

The two ALCO-built 4-6-6-4s received in 1944 were also oil burners and were designated Class Z-8 and given road numbers 910 and 911. These locomotives had (4) 23 x 32 cylinders, 70" drivers, a 260 psi boiler pressure, a tractive effort of 107,000 lbs and weighed 639,000 pounds.

Railroads that used 4-6-6-4 "Challenger" Locomotives in the USA (data provided by Steve Llanso of Sweat House Media)

These larger engines allowed the Clinchfield to move longer and heavier coal trains across the mountains. The Challengers were the largest and most memorable. Clinchfield purchased the Challengers between 1942 and 1947, but all were retired by 1954 due to Clinchfield being an early adopter of an all diesel fleet.

2025年3月24日 · UP tested the new 4-6-6-4 engine on a section of track in Utah over the Wahsatch Range. The engineer commented that would be a challenge for any locomotive, and the name stuck.

The 4-6-6-4 was one of the most technologically advanced of its kind with advancements that allowed for both high horsepower and efficient operations. Sadly, however, like the Yellowstones the Challengers had an extremely short lifespan.