For describing the wheel arrangement of diesel and electric locomotives in North America, the American Association of Railroads has established a notation system. Commonly referred to as AAR wheel arrangement, it is quite different from the
Whyte notation used for describing the wheel arrangements of steam locomotives. The AAR notation is a subset, or simplification, of the European UIC notation.
AAR Notation
In the AAR notation numerals indicate non-powered axels and corresponding letters indicate powered axels. So values are as follows:
1- One unpowered axel.
A - One powered axel.
2 - Two unpowered axels.
B - Two powered axels.
3 - Three unpowered axels.
C - Three powered axels.
4 - Four unpowered axels.
D - Four powered axels.
Non-Articulated Locomotives
The axle configurations of locomotive trucks are expressed in groups of one or more characters. A dash character (-) is used to separate trucks in writing a locomotive's entire wheel configuration. Here are some common examples:
B-B
B-B denotes a four axle two truck locomotive where each truck has two powered axles. This may well be the most common of all wheel configurations. EMD F and GP units were B-B locomotives.A1A-A1A
A1A-A1A denotes a six axle two truck locomotive where each truck has one unpowered axle interleaved between two powered axles. EMD E Units and Alco PA and PB locomotives used this wheel configuration.C-C
C-C denotes a two truck locomotive where each truck has three powered axles. This is the wheel configuration of the popular EMD SD series of locomotives.Articulated Locomotives
A plus character (+) is used to indicate articualted locomotives, or locomotives with permanently coupled power units.B-B+B-B
B-B+B-B indicates two power units with B-B trucks permanently coupled. The Pullman/EMD streamlined M-10003 through M-10006 trains were powered by this configuration of locomotives.
