The AAR wheel arrangement system is a method of classifying locomotive wheel arrangements that was developed by the Association of American Railroads. Essentially a simplification of the European UIC classification, it is widely used in North America to describe diesel and electric locomotives. It is not used for steam locomotives, which use the Whyte notation instead.
In the Whyte notation, a 6-2-0 is a railroad steam locomotive that has an unpowered three-axle leading truck followed by a single powered driving axle. This wheel arrangement is associated with the Crampton locomotive type, and in the USA the single class were sometimes referred to as Cramptons.
A 4-4-4-4 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, has a four-wheel leading truck, two sets of four driving wheels, and a four-wheel trailing truck. While it would be possible to make an articulated locomotive of this arrangement, the only 4-4-4-4s ever built were duplex locomotives—with two sets of cylinders driving two sets of driven wheels in one rigid frame, essentially a 4-8-4 with divided drive.
A 6-4-4-6 steam locomotive, in the Whyte notation for describing locomotive wheel arrangements, is one with six leading wheels, two sets of four driving wheels, and six trailing wheels.
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, a booster engine was mounted on the trailing truck to provide extra tractive effort when starting a heavy train and at low speeds on gradients.
The Pennsylvania Railroad's class P5 comprised 92 mixed-traffic electric locomotives constructed 1931–1935 by the PRR, Westinghouse and General Electric. Although the original intention was that they work many passenger trains, the success of the GG1 locomotives meant that the P5 class were mostly used on freight. A single survivor, prototype #4700, is at the National Museum of Transportation in St Louis, Missouri.
The Pennsylvania Railroad's class O1 comprised eight experimental boxcab electric locomotives built in 1930 and 1931. They were built in preparation for the New York to Washington Electrification project. They had the wheel arrangement classified as 4-4-4 in the Whyte notation. Although successful, they were not powerful enough for the railroad's increasingly heavy trains. For production, the PRR chose to concentrate on the P5 class, effectively an enlarged and more powerful version of the O1 with an additional pair of driving wheels.
The Pennsylvania Railroad's class DD2 electric locomotive was a single prototype locomotive never placed into series production. It was intended as an improved and simplified GG1 for use on the planned, but never built, extension of the PRR's electrification west of Harrisburg, Pennsylvania. The one locomotive produced was numbered #5800 and used in regular Baltimore tunnel helper service until it was scrapped in September 1962.
The Pennsylvania Railroad's class FF1 was an American electric locomotive, a prototype numbered #3931 and nicknamed "Big Liz". It was built in 1917 to haul freight trains across the Allegheny Mountains where the PRR planned to electrify. "Big Liz" proved workable but too powerful for the freight cars of the time with its 4600 available horsepower and astonishing 140,000 lbf (620 kN) of tractive effort. Pulling the train it regularly snapped couplers and when moved to the rear as a pusher its force was sufficient to pop cars in the middle of the train off the tracks.
Pennsylvania Railroad class E3b was an experimental electric locomotive supplied by Westinghouse Electric & Manufacturing Co. The locomotive was of the two unit design, with each unit having a B-B-B (AAR) or Bo-Bo-Bo (UIC) wheel arrangement. The bodywork and running gear was produced by Baldwin-Lima-Hamilton while the electrical equipment was provided by Westinghouse, who also acted as the main contractor.
Pennsylvania Railroad class E3c comprised a pair of experimental C-C (AAR) or Co-Co (UIC) electric locomotives. The bodywork and running gear was produced by Baldwin-Lima-Hamilton while the electrical equipment was provided by Westinghouse, who also acted as principal contractor.
The Pennsylvania Railroad's class R1 comprised a single prototype electric locomotive constructed in 1934 by the Baldwin Locomotive Works of Philadelphia, Pennsylvania, USA, with the electrical equipment by Westinghouse.
The Pennsylvania Railroad's class AA1 comprised two experimental electric locomotives constructed in 1905 by the company's own Altoona Works with the assistance of Westinghouse. Intended as testbeds as the PRR began its electrification project, both locomotives remained service into the 1930s.
The Pennsylvania Railroad class Q1, #6130, was a single experimental steam locomotive designed for dual service. The locomotive entered service in 1942, and retired in 1949 after accumulating a relatively low 165,000 service miles.
In Whyte notation, a 4-6-6-2 is a steam locomotive with four leading wheels in an unpowered bogie at the front of the locomotive followed by two sets of driving wheels with six wheels each, followed by two unpowered trailing wheels at the rear of the locomotive.
The class D15 of the Pennsylvania Railroad comprised a solitary Lindner-system cross compound steam locomotive of 4-4-0 "American" wheel arrangement in the Whyte notation. The sole locomotive was #1515, built in 1892 at the PRR's Altoona Shops, it had very British lines with a full-length footplate, splashers, a six-wheel tender, and large 84 in (2,134 mm) drivers. It was built in 1892 by the PRR’s Altoona Works it remained in service until it was retired in 1945 and put in storage at the PRR’s Altoona Works and was scrapped in 1951.
Class D14 on the Pennsylvania Railroad was a type of steam locomotive with a 4-4-0 wheel arrangement in the Whyte notation. They were originally designated class P in the PRR's pre-1895 classification scheme. Twenty-two locomotives were built at the PRR's Altoona Works ; six in 1893 with 78-inch (1,981 mm) driving wheels, and sixteen in 1894 with 80-inch (2,032 mm) drivers, classified D14a. Later, all sixteen class D14a were rebuilt to class D14b with 68-inch (1,727 mm) drivers for secondary service after they were replaced in top-flight express service, while three of the six class D14 were similarly rebuilt to class D14c.
The Pennsylvania Railroad's class A5s was the largest class of 0-4-0 steam locomotives. The Pennsylvania Railroad built 47 in its Juniata Shops between 1916–1924. They were all retired by 1957. One is preserved at the Railroad Museum of Pennsylvania.
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.
Under the Whyte notation for the classification of steam locomotives, 0-6-6 represents the wheel arrangement of no leading wheels, six powered and coupled driving wheels on three axles and six trailing wheels on three axles.
