The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte, and came into use in the early twentieth century following a December 1900 editorial in American Engineer and Railroad Journal. The notation counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, numbers being separated by dashes. Other classification schemes, like UIC classification and the French, Turkish and Swiss systems for steam locomotives, count axles rather than wheels.
A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as multiple units, motor coaches, railcars or power cars; the use of these self-propelled vehicles is increasingly common for passenger trains, but rare for freight.
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 to help the locomotive negotiate curves and to support the front portion of the boiler.
This wheel arrangement was proposed by Lima Locomotive Works in 1949 as a continuation of their "Super Power" concept, essentially an expansion of the 4-8-4. A larger firebox similar to the ones on the 2-6-6-6 locomotives built by Lima would have been fitted, allowing for greater power at speed. Despite promotion by Lima, there is no firm evidence that an example of this type was ever built and no nickname was ever assigned to the arrangement. In 1949, few railroads were interested in new steam locomotives due to steady improvements in diesel-electric locomotives.
Lima Locomotive Works was an American firm that manufactured railroad locomotives from the 1870s through the 1950s. The company took the most distinctive part of its name from its main shop's location in Lima, Ohio. The shops were located between the Baltimore & Ohio's Cincinnati-Toledo main line and the Nickel Plate Road main line and shops.
"Superpower steam" was a term coined by Lima Locomotive Works in the mid-1920s. It referred to steam locomotives with booster-equipped four-wheel trailing trucks supporting large fireboxes, as well as enlarged superheaters. The wheel arrangements introduced in the 1920s for these locomotives: the 4-6-4's, 2-8-4's, 4-8-4's and 2-10-4's, and in the 1930s, the 2-6-6-4's. The term "superpower" was often applied to all locomotives with 4 wheel trailing truck arrangements afterward, though many did not have boosters and almost all steam of any wheel arrangement built after that time had large superheaters. The design was invented by Lima for the New York Central's Boston and Albany Railroad. The design was invented by Lima's Vice President of Engineering, William Woodard. The first "superpowered" steam locomotive was a 2-8-4 steam locomotive that essentially an expanded 2-8-2 Mikado. The new design called for double the firebox size than the earlier Mikados, thus giving it more grate area. The larger grate area meant that the locomotive could achieve greater steaming capacity, higher speeds, more horsepower, and greater tractive effort. To accommodate the larger firebox, Woodard gave the new locomotive a two axle trailing truck instead of the common single axle trailing trucks previously used. The design was a smashing success and soon both the Baldwin Locomotive Works, the American Locomotive Company followed suit.
Under the Whyte notation for the classification of steam locomotives, 4-8-4 represents the wheel arrangement of four leading wheels on two axles, eight powered and coupled driving wheels on four axles and four trailing wheels on two axles. The type was first used by the Northern Pacific Railway, and initially named the Northern Pacific, but railfans and railroad employees have shortened the name when referring to the type, and now is most commonly known as a Northern.
It is possible that CB&Q 4-8-4 5601 was experimentally equipped with a six-wheel trailing truck to allow use on branch lines with lighter rail, but the experiment was unsuccessful. Photographic evidence is said to have existed in the 1950s, but no photos are known to exist at present.
Despite there being no documented full size examples built, there have been some 4-8-6s built for model railroads.
Reference:
Bill Withuhn, "Did we scrap steam too soon?", in Trains, Vol. 34, No. 8, June 1974, pp.36–47.
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Under the Whyte notation for the classification of steam locomotives, a 2-10-4 locomotive has two leading wheels on one axle, usually in a bissel truck, ten coupled driving wheels on five axles, and four trailing wheels on two axles, usually in a bogie. These were referred to as the Texas type in most of the United States, the Colorado type on the Burlington Route and the Selkirk type in Canada.
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Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 6-8-6 represents the arrangement of six unpowered leading wheels arranged into a three-axle leading truck, eight powered driving wheels, and six unpowered trailing wheels arranged into a three-axle trailing truck.
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