The AAR (Association of American Railroads) classification is a system for explaining the wheel arrangements of locomotives and multiple units. The system is a simplification of the European version.
These are the rules of the code:
- Capital letters denote powered axles: A = 1 powered axle; B = 2; C = 3 etc.
- A number denotes a non-powered axle.
- A dash ( - ) denotes the end of a truck, and/or the beginning of another.
- A plus sign ( + ) denotes articulation.