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 Classification of Diesel Locomotives.

The AAR wheel arrangement system is a method of classifying diesel-electric locomotive (or unit) wheel arrangements that was developed by the Association of American Railroads. It is essentially a simplification of the European UIC classification method. It is widely used in North America to describe diesel-electric locomotives. This system counts axles instead of wheels. Letters refer to powered axles, and numbers to un-powered (or idler) axles. "A" refers to one powered axle, "B" to two powered axles in a row, "C" to three powered axles in a row, and "D" to four powered axles in a row. "1" refers to one idler axle, and "2" to two idler axles in a row. A dash "-" separates trucks, or wheel assemblies. A plus sign "+" refers to articulation. Some common Canadian classifications are shown below:

A Budd Rail Diesel Car (RDC) is 1A-A1.
1A-A1 - means there are two trucks (or wheel assemblies) under the unit. Each truck has one powered axle, and one idler axle, with the idler axles to the outside.

A General Motors E8A is classified as A1A-A1A.
A1A-A1A - means there are two trucks under the unit. Each truck has one powered axle, one idler axle, and one more powered axle. This was designed to spread the weight of the locomotive evenly over the track. Examples include the EMD E-units and ALCO PAs, which were passenger locomotives, and the dual service FM Erie-built. BNSF is scheduled to take delivery of ES44C4 locomotives with this type of truck in 2009.

A General Motors GP9 is classified as B-B.
B-B - means there are two identical trucks. Each truck has two powered axles, a currently-popular configuration used in high-speed, low-weight applications, such as intermodal trains, high-speed rail, as well as switching. Examples include the General Motors GP (General Purpose) units.

A General Electric AC4400CW is classified as C-C.
C-C means there are two identical trucks. Each truck has three powered axles. Examples include the EMD SD (Special Duty) units. This is a currently popular configuration used in low-speed, high-weight applications, such as unit coal trains.