AAR wheel arrangement
method of classifying locomotive wheel arrangements
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 anywhere, which use the Whyte notation instead, except geared steam locomotives, which are instead classified by their model and their number of trucks (bogies in Commonwealth English). The AAR system (like UIC) counts axles, unlike Whyte notation, which counts wheels.
Structure
Letters
Letters refer to powered axles, with 'A' representing one powered axle, 'B' two consecutive powered axles on the same truck (bogie), 'C' three consecutive powered axles on the same truck, and so on.
Numbers
Numbers are used to show unpowered (idler) axles. "1" refers to one idler axle, and "2" to two idler axles in a row on the one truck (bogie), and so on.
Symbols
A dash ("–") separates trucks (bogies) 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, or in trainsets and multiple units, where each unit is permanently coupled to the other.
“AAR wheel arrangement” enters the record as method of classifying locomotive wheel arrangements. Crown Archives preserves that source wording while asking what wheel, arrangement and method can confirm, complicate or overturn.
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This entry incorporates text from “AAR wheel arrangement” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.