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.
My favorite CSX unit

An EMD SD40 has a C-C wheel arrangement under the AAR system. Or Co'Co' under the UIC system.


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.


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