A 4-8-8-4 in the

Whyte notation Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth cen ...

for the classification of steam locomotives by

wheel arrangement In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and c ...

, is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel

trailing truck On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle ( wheelset) located behind the driving wheels. The axle of the trailing wheels is usually located in a trailing truck. On some large locomotives, ...

. Only one model of locomotives has ever used this configuration, and that is commonly known as "

Union Pacific Big Boy The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1962. The 25 Big ...

s" after the railroad that operated them. Other equivalent classifications are: *

UIC classification The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...

: '2DD2' (also known as German classification and Italian classification) * French locomotive classification: '240+042' * Turkish locomotive classification: '46+46' *

Swiss classification For more than a century, the Swiss locomotive, multiple unit, motor coach and railcar classification system, in either its original or updated forms, has been used to name and classify the rolling stock operated on the railways of Switzerland. ...

: '4/6+4/6' The equivalent UIC classification is refined to (2′D)D2′ for

Mallet locomotive The Mallet locomotive is a type of articulated steam railway locomotive, invented by the Swiss engineer Anatole Mallet (1837–1919). The front of the locomotive articulated on a bogie. The compound steam system fed steam at boiler pressur ...

s. 4-8-8-4 Big Boys were only produced for the

Union Pacific Railroad The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Paci ...

. Twenty-five such engines were built between 1941 and 1944, numbered 4000 to 4024. Eight of these locomotives survive, seven of which are on static public display at various sites in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. Union Pacific announced plans in August 2013 to restore No. 4014 to operation for use in mainline excursion service. No. 4014 underwent restoration between 2016 and 2019, and completed its first post-restoration test run on May 2, 2019. The UP Big Boys were an expansion of the

4-6-6-4 In Whyte notation, a 4-6-6-4 is a railroad steam locomotive that has four leading wheels followed by six coupled driving wheels, a second set of six driving wheels and four trailing wheels. 4-6-6-4's are commonly known as Challengers. Other eq ...

"Challenger" type articulated locomotive. Adding four driving wheels increased the pulling power of the locomotive and reduced the need for helper locomotives over steep grades. Although their wheels were an inch (25.4 mm) smaller than those of the Challengers, they were still able to sustain similar high speeds. Other American railroads considered buying 4-8-8-4s, including the

Western Pacific Railroad The Western Pacific Railroad was a Class I railroad in the United States. It was formed in 1903 as an attempt to break the near-monopoly the Southern Pacific Railroad had on rail service into northern California. WP's Feather River Route dire ...

, which already rostered large 2-8-8-2s and 4-6-6-4s, but diesel locomotives were gaining popularity and soon were able to displace these Big Boy locomotives.

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