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; instead, the Whyte notation is used. This 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 ("–") separates trucks, or wheel assemblies. A plus sign ("+") refers to articulation.
A1A-3
"A1A-3" means there are two trucks, or wheel assemblies. The "A1A" truck is under the front of the unit, and has one powered axle, one idler axle, and one more powered axle. The "3" truck is under the back of the unit, and has three idler axles in a row. An example would be the Baldwin 1,000 hp (750 kW) cab unit DR-6-2-10 ; one example for the Chicago and Northwestern Railway was built in 1948.
A1A-A1A
"A1A-A1A" means there are two identical trucks, or wheel assemblies under the unit. Each truck has one powered axle, one idler axle, and one more powered axle. This was to more evenly spread the weight of the locomotive over the track. Examples would be the EMD E-units, which were passenger locomotives.
B
"B" means there are two powered axles under the unit. These axles are not articulated relative to other parts of the locomotive. This arrangement is only used on very small locomotives.
B-1
"B-1" means there are two trucks, or wheel assemblies. The "B" truck is under the front of the unit, and has two powered axles. The "1" truck is under the back of the unit, and has one idler axle. Examples are the three LWT12 (Aerotrain) locomotives built by EMD in 1956.
B-2
"B-2" means there are two trucks, or wheel assemblies. The "B" truck is under the front of the unit, and has two powered axles. The "2" truck is under the back of the unit, and has two idler axles in a row. Examples are the three lightweight locomotives built by Baldwin in 1956 and 1957 for use with Pullman-Standard Train X equipment.
B-A1A
"B-A1A" means there are two trucks, or wheel assemblies. The "B" truck is under the front of the unit, and has two powered axles. The "A1A" truck is under the back of the unit, and has one powered axle, one idler axle, and one more powered axle. Examples would be the Fairbanks-Morse passenger C-liners, built from 1950-1955 and the EMD_FL9.
B-B
"B-B" means there are two identical trucks, or wheel assemblies under the unit. Each truck has two powered axles. Examples would be the EMD GP (General Purpose) units. This is a currently popular configuration used in high-speed, low-weight applications, such as intermodal trains.
B-B-B
"B-B-B" means there are three identical trucks, or wheel assemblies under the unit. Each truck has two powered axles. This arrangement is unusual. Placing six axles in a C-C arrangement is far more common. The EMD GM10 electric locomotive used this arrangement.
B+B-B+B
"B+B-B+B" means there are four identical trucks, or wheel assemblies under the unit. Within each truck, there are two powered axles, and pairs of them are connected by span bolsters. An example would be the General Electric U50, built from 1963-1965. The 4500 horsepower (3.4 MW) turbine locomotives built by GE for Union Pacific also used this arrangement.
B-B+B-B
"B-B+B-B" means that the locomotive has four identical trucks, or wheel assemblies. Each truck contains two powered axles. The middle pair of trucks are connected by a span bolster. In most cases the locomotive is articulated over the span bolster. The Union Pacific's M-10002 is an example of this type.
C-C
"C-C" means there are two identical trucks, or wheel assemblies under the unit. Each truck has three powered axles. Examples would be the EMD SD (Special Duty) units. This is a currently popular configuration used in low-speed, high-weight applications, such as unit coal trains.
2-C+C-2
"2-C+C-2" means there are four trucks, or wheel assemblies under the unit. At either end are trucks with two idler axles. In the center there are two trucks with three powered axles each. The GG1 electric locomotives and GE steam turbine electric locomotives used this arrangement.
C-C+C-C
"C-C+C-C" means there are four identical trucks, or wheel assemblies under the unit. Each truck has three powered axles. The only examples of this type were the 8500 horsepower (6.3 MW) gas turbine locomotives built by General Electric for Union Pacific. These locomotives consisted of two permanently coupled units, each with two trucks.
C+C-C+C
"C+C-C+C" means there are four identical trucks, or wheel assemblies under the unit. Each truck has three powered axles and pairs of them are connected by span bolsters. This was used on the "Jawn Henry" steam turbine-electric locomotive.
2-D-2
"2-D-2" means there are three trucks, or wheel assemblies under the unit. At either end are trucks with two idler axles. The center truck has four powered axles. The PRR R1 electric locomotive used this arrangement.
D-D
"D-D" means there are two identical trucks, or wheel assemblies under the unit. Each truck has four powered axles. Examples are the EMD DD (Double Diesel) units.
2-D+D-2
"2-D+D-2" means there are two sets of articulated axles under the unit. Within each of these sets, there is a truck of two idler axles at the ends of the locomotive, and inboard of each is a truck of four powered axles in a row. Examples include the Baldwin DR-12-8-1500/2 "Centipede" diesel locomotives and the GE Little Joe electric locomotives.
B-D+D-B
"B-D+D-B" means there are two sets of articulated axles under the unit. Within each of these sets, there is a truck with two powered axles at the ends of the locomotive, and inboard of each of these is a truck with four powered axles. An example of this is the W1 class of electric locomotives built by General Electric for the Great Northern.
References
- Pinkepank, Jerry A. (1973). The Second Diesel Spotter’s Guide, pp.8–9, and other pages. Kalmbach Books. Library of Congress Catalog Card No. 66–22894.
