the aar wheel arrangement system is a method of classifying locomotive (or unit) wheel arrangements that was developed by the association of american railroads. it is essentially a simplification of the european uic classification, and 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. 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 ("&ndash;") separates trucks, or wheel assemblies. a plus sign ("+") refers to articulation. ==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. examples include budd rdc diesel mu (dmu) cars. ==1-d== "1-d" means there are two trucks. the "1" truck is under the front of the unit, and has one idler axle. the remaining 4 axles are rigidly mounted to the frame behind this lead truck . the only known examples are a series of diesel locomotives built and owned by the texas mexican railway. ==2-a1a== "2-a1a" means there are two trucks. the "2" truck is under the front of the unit, and has two idler axles in a row. the "a1a" truck is under the rear of the unit, and has one powered axle, one idler axle, and one more powered axle. an example is the fm op800  railcar, six of which were built by the st. louis car company exclusively for the southern railway in 1939. ==2-b== "2-b" means there are two trucks, or wheel assemblies. the "2" truck is under the front of the unit, and has two idler axles in a row. the "b" truck is under the rear of the unit, and has two powered axles. examples include the three lightweight power cars built by alco/acf in 1935 and 1937 for use with the rebel streamliners. ==3-a1a== "3-a1a" means there are two trucks. the "3" truck is under the front of the unit, and has three idler axles in a row. the "a1a" truck is under the rear of the unit, and has one powered axle, one idler axle, and one more powered axle. an example is the later built fm op800  railcar, six of which were built by the st. louis car company exclusively for the southern railway in 1939. ==a1-1a== "a1-1a" means the
Locomotive_classification_systems