AAR wheel arrangement

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.

Legend

Letter or number	Definition
"A"	one powered axle
"B"	two powered axles in a row
"C"	three powered axles in a row
"D"	four powered axles in a row
"1"	one idler axle
"2"	two idler axles in a row
"3"	three idler axles in a row
A dash ("–") separates trucks, or wheel assemblies.
A plus sign ("+") refers to articulation.

A1A-2

"A1A-2" - 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 "2" truck is under the back of the unit, and has two idler axles in a row. The only documented example is the FM OP800 800 hp railcar, six of which were built by the St. Louis Car Company exclusively for the Southern Railway in 1939.

A1A-3

"A1A-3" - 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 is the Baldwin DR-6-2-10 1,000 hp cab unit, only one of which was built for the Chicago and North Western Railway in 1948.

A1A-A1A

"A1A-A1A" - 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 include the EMD E-units and ALCO PAs, which were passenger locomotives, and the dual service FM Erie-built.

A1A-B+B

"A1A-B+B" - three trucks, or wheel assemblies under the unit. The first truck has three axles, with the center one unpowered. A pair of two-axle trucks, each with both axles powered, are connected by a span bolster under the rear of the unit. The only example to date of this arrangement was a single experimental EMD SDP45.

B

"B" - 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, such as the EMD Model 40. This arrangement is sometimes referred to as 0-4-0, the Whyte notation equivalent.

B-1

"B-1" - 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 include the three EMD LWT12 Aerotrain locomotives built by EMD in 1956.

B-2

"B-2" - 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 include the three lightweight locomotives built by Baldwin in 1956 and 1957 for use with Pullman-Standard Train X equipment.

B-A1A

"B-A1A" - 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 include the FM C-liners (most passenger units) built from 1950–1955, and the EMD FL9.

B-B

"B-B" - two identical trucks, or wheel assemblies under the unit. Each truck has two powered axles, a currently-popular configuration used in high-speed, low-weight applications, such as intermodal trains. Examples include the EMD GP (General Purpose) units.

B-B-B

"B-B-B" - three identical trucks, or wheel assemblies under the unit. Each truck has two powered axles. The locomotive frame must either be articulated or significant side play must be provided to the center truck. See also Bo-Bo-Bo.

2-B+B-2

"2-B+B-2" - two sets of articulated axles under the unit. Within each of these sets, there is a truck with two idler axles, and inboard of it are two powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The PRR DD1 and DD2 electric locomotives used this arrangement.

B+B-B+B

"B+B-B+B" - 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. The EVFM railway of Brazil uses narrow gauge GE "BB" locomotives with this arrangement, both with "standard" and wide cabs. A GE BB40-9W, for instance, is a wide cab GE Dash 9-40CW series 4000 hp locomotive with a B+B-B+B wheel arrangement.

B-B+B-B

"B-B+B-B" - 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. This arrangement also includes locomotives made of two permanently coupled B-B units, such as some EMD FT units which had a solid drawbar connecting two units instead of the typical couplers.

B-B+B-B+B-B

"B-B+B-B+B-B" - the locomotive has six identical trucks, or wheel assemblies. Each truck contains two powered axles. The only known locomotives to have this configuration were the two EMD TR3 locomotives made of three permanently coupled B-B units, which had solid drawbars connecting the units instead of the typical couplers.

C

"C" - three powered axles under the unit. They are not articulated relative to other parts of the locomotive. This arrangement is only used on very small locomotives. This arrangement is sometimes referred to as 0-6-0, the Whyte notation equivalent.

C-C

"C-C" - two identical trucks, or wheel assemblies under the unit. Each truck has three powered axles. Examples include the EMD SD (Special Duty) units. This is a currently popular configuration used in low-speed, high-weight applications, such as unit coal trains. See also Co-Co.

1-C+C-1

"1-C+C-1" - two sets of articulated axles under the unit. Within each of these sets, there is a truck with one idler axle, and inboard of it are three powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The PRR FF1 and FF2 electric locomotives used this arrangement.

2-C+C-2

"2-C+C-2" - two sets of articulated axles under the unit. Within each of these sets, there is a truck with two idler axles, and inboard of it are three powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The GG1 electric locomotives were notable examples of this arrangement.

2-C1+2-C1-B

"2-C1+2-C1-B" - five trucks. Only the first three axles on the four-axle trucks were powered, as were both axles in the last truck; the first and middle trucks had two unpowered axles each. The only examples of this arrangement were three unique coal-fired steam-turbine locomotives built by the Baldwin Locomotive Works for the Chesapeake and Ohio Railway between 1947 and 1948.

C-C+C-C

"C-C+C-C" - 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) turbine locomotives built by General Electric for Union Pacific. These locomotives consisted of two permanently-coupled C-C units.

C+C-C+C

"C+C-C+C" - 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 arrangement was used on the Jawn Henry coal-fired steam-turbine locomotive built by the Baldwin Locomotive Works for the Norfolk & Western Railway in May, 1954.

2-D-2

"2-D-2" - 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" - two identical trucks, or wheel assemblies under the unit. Each truck has four powered axles. Examples include the EMD DD units.

2-D+D-2

"2-D+D-2" - two sets of articulated axles under the unit. Within each of these sets, there is a truck with two idler axles, and inboard of it are four powered axles. Two of these articulated sets are placed back to back and connected by a hinge. 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" - two sets of articulated axles under the unit. Within each of these sets, there is a truck with two powered axles, and inboard of it are four powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The W1 class of electric locomotives built by General Electric for the Great Northern Railway used this arrangement.

1B-D+D-B1

"1B-D+D-B1" - two sets of articulated axles under the unit. Within each of these sets, there is a truck with one unpowered axle and two powered axles, and inboard of there are four powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The "Bi-Polar" electric locomotives used by the Milwaukee Road used this arrangement.

See also

• UIC classification
• Wheel arrangement
• Whyte notation
• Category:Locomotives by wheel arrangement

References

• Pinkepank, Jerry A. (1973). The Second Diesel Spotter's Guide. Milwaukee, WI: Kalmbach Publishing. ISBN 0-89024-026-4.