In the

Whyte notation The 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 twenti ...

for classifying steam locomotives by wheel arrangement, a 4-6-6-4 is a

railroad Rail transport (also known as train transport) is a means of transport using wheeled vehicles running in railway track, tracks, which usually consist of two parallel steel railway track, rails. Rail transport is one of the two primary means of ...

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

that has four

leading wheel The leading wheel or leading axle or pilot wheel of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located on a leading truck. Leading wheels are used ...

s followed by two sets of six coupled

driving wheel On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled t ...

s and four

trailing wheel On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle (Wheelset (rail transport), wheelset) located behind the driving wheels. The axle of the trailing wheels is usually located in a trailing Bogie, t ...

s. 4-6-6-4's are commonly known as Challengers. A similar wheel arrangement exists for

Garratt locomotive A Garratt locomotive is a type of articulated steam locomotive invented by the engineer Herbert William Garratt that is articulated into three parts. Its boiler, firebox, and cab are mounted on a centre frame or "bridge". The two other p ...

s, on which both engine units swivel, but is referred to as 4-6-0+0-6-4. 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'', Cr ...

: 2CC2 (also known as German classification and

Italian 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'', Cr ...

)

French classification Under the French classification system for locomotive wheel arrangements, the system is slightly different for steam and electric/diesel vehicles. Steam The French system counts axles, rather than wheels. As with Whyte notation, a conventional r ...

: 230+032
Turkish classification: 35+35

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. ...

: 3/5+3/5 The

is refined to (2'C)C2' for simple

articulated locomotive An articulated locomotive is a steam locomotive (rarely, an electric locomotive) with one or more engine units that can move independently of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to ...

s. Challengers were most common in the

Union Pacific Railroad The Union Pacific Railroad is a Railroad classes, Class I freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United Stat ...

, but many other railroads ordered them as well. An expansion for the

Union Pacific Challenger The Union Pacific Challengers are a type of simple articulated 4-6-6-4 steam locomotive built by American Locomotive Company (ALCO) from 1936 to 1944 and operated by the Union Pacific Railroad until the late 1950s. A total of 105 Challengers w ...

class was the

Union Pacific Big Boy The Union Pacific Big Boy is a type of simple articulated locomotive, 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 ...

class, being a

4-8-8-4 A 4-8-8-4 in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck. Only one model of locomotives ha ...

, instead of a 4-6-6-4. Today, the only Challenger locomotives that survive were both owned by Union Pacific. One such locomotive,

Union Pacific 3985 Union Pacific 3985 is a four-cylinder Articulated locomotive, articulated 4-6-6-4 "Union Pacific Challenger, Challenger"-type steam locomotive built in July 1943 by the American Locomotive Company (ALCO) of Schenectady, New York, for the Union P ...

, was operated by the Union Pacific Railroad in excursion service from 1981 to 2010, when mechanical problems took it out of service. It was retired in January 2020 due to its poor mechanical condition and subsequently donated to the Railroading Heritage of Midwest America, where it is now undergoing a second restoration. The second example, Union Pacific 3977 is on static display in Cody Park

North Platte, Nebraska North Platte is a city in and the county seat of Lincoln County, Nebraska, United States. It is located in the west-central part of the state, along Interstate 80, at the confluence of the North and South Platte Rivers forming the Platte River. ...

. Though originally intended for freight service, many units could be found for leading passenger consists as well.

Roster of locomotives

References

External links