AAR wheel arrangement

The AAR wheel arrangement system is a method of classifying locomotive (or unit) 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 (including third-rail electric locomotives). It is not used for steam locomotives, (except geared steam locomotives, which are instead classified by their model and their number of trucks), which use the Whyte notation instead.

The AAR system counts axles instead of wheels. Letters refer to powered axles, and numbers to unpowered (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, either by connecting bogies with span bolsters or by connecting individual locomotives via solid drawbars instead of couplers.