The AAR wheel arrangement system is a method of classifying locomotive (or unit)

wheel arrangement In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and ...

s that was developed by the

Association of American Railroads The Association of American Railroads (AAR) is an industry trade group representing primarily the major freight Rail transport, railroads of North America (Canada, Mexico and the United States). Amtrak and some regional Commuter rail in North Am ...

. Essentially a simplification of the European

UIC classification The UIC classification of locomotive axle arrangements, sometimes known as the German classification''The Railway Data File''. Leicester: Silverdale, 2000. p. 52. . or German system,Kalla-Bishop P.M. & Greggio, Luciano, ''Steam Locomotives'', Cre ...

, it is widely used in North America to describe diesel and

electric locomotive An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a battery or a supercapacitor. Locomotives with on-board fuelled prime movers, such as diesel engines or ga ...

s (including third-rail electric locomotives). It is not used for

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the loco ...

s, which use the

Whyte notation Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It was devised by Frederick Methvan Whyte, and came into use in the early twentieth cen ...

instead. The AAR 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 A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...

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.

1A-A1

"1A-A1" means there are two trucks (or wheel assemblies) under the unit. Each truck has one powered axle and one idler axle, with the idler axles to the outside. Examples include

Budd RDC The Budd Rail Diesel Car, RDC, Budd car or Buddliner is a self-propelled diesel multiple unit (DMU) railcar. Between 1949 and 1962, 398 RDCs were built by the Budd Company of Philadelphia, Pennsylvania, United States. The cars were primarily ado ...

Diesel MU ( DMU) cars.

1B-1B

"1B-1B" means there are two trucks with a leading idler axle in front of two powered axles. This arrangement was used to upgrade the B-B arrangement of two EMC 1800 hp B-B locomotives owned by the

Santa Fe Railroad The Atchison, Topeka and Santa Fe Railway , often referred to as the Santa Fe or AT&SF, was one of the larger railroads in the United States. The railroad was chartered in February 1859 to serve the cities of Atchison and Topeka, Kansas, and S ...

in 1938, for greater stability at speed.

1-D

"1-D" means there are two trucks or groups of axles; the "1" truck is under the front of the unit, and has one idler axle. The remaining 4 axles are rigidly mounted to the frame behind this lead truck (or grouped in a second truck). This is roughly the equivalent of a 2-8-0 Consolidation in the Whyte notation, particularly when built as a 1-truck/4 rigid axle locomotive. The only known examples are a series of diesel boxcab locomotives built and owned by the Texas Mexican Railway.Texas-Mexican 701A
/ref>

2-A1A

"2-A1A" means there are two trucks or wheel assemblies. The "2" truck is under the front of the unit, and has two idler axles in a row. The "A1A" truck is under the rear of the unit, and has one powered axle, one idler axle, and one more powered axle. An example is the

FM OP800 The OP800 was a lightweight, streamlined railcar built by the St. Louis Car Company in 1939. Fairbanks-Morse supplied the , five-cylinder opposed piston engine prime mover. The units were configured in a highly unusual 2-A1A wheel arrangement ...

railcar, six of which were built by the St. Louis Car Company exclusively for the Southern Railway in 1939.

2-B

"2-B" means there are two trucks or wheel assemblies. The "2" truck is under the front of the unit, and has two idler axles in a row. The "B" truck is under the rear of the unit, and has two powered axles. Examples include the three lightweight

power car In rail transport, the expression power car may refer to either of two distinct types of rail vehicle: *a vehicle that propels, and commonly also controls, a passenger train, multiple unit or tram, often as the lead vehicle; *a vehicle equipped ...

s built by

ALCO The American Locomotive Company (often shortened to ALCO, ALCo or Alco) was an American manufacturer of locomotives, diesel generators, steel, and tanks that operated from 1901 to 1969. The company was formed by the merger of seven smaller locom ...

/ ACF in 1935 and 1937 for use with the Rebel streamliners.

3-A1A

"3-A1A" means there are two trucks or wheel assemblies. The "3" truck is under the front of the unit, and has three idler axles in a row. The "A1A" truck is under the rear of the unit, and has one powered axle, one idler axle, and one more powered axle. An example is the later built

railcar, six of which were built by the St. Louis Car Company exclusively for the Southern Railway in 1939.

A1-1A

"A1-1A" means there are two trucks or wheel assemblies under the unit. Each truck has one powered axle and one idler axle, with the powered axles to the outside.

A1A-2

"A1A-2" means there are two trucks. 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. An example is the ''Silver Charger''

for the General Pershing Zephyr.

A1A-3

"A1A-3" means there are two trucks. 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 cab unit, only one of which was built for the

Chicago and North Western Railway The Chicago and North Western was a Class I railroad in the Midwestern United States. It was also known as the "North Western". The railroad operated more than of track at the turn of the 20th century, and over of track in seven states befo ...

in 1948.

A1A-A1A

"A1A-A1A" means there are two trucks under the locomotive. Each truck has two powered axles, with an idler axle between them. This spreads the weight of the locomotive more evenly over the track and counteracts the tendency of trucks to oscillate at high speeds, which is a problem with two axle trucks. The idler wheels may be smaller than the powered wheels. Examples of locomotives with this wheel arrangement include the EMD E-units and

ALCO PA The ALCO PA was a family of A1A-A1A diesel locomotives built to haul passenger trains. The locomotives were built in Schenectady, New York, in the United States, by a partnership of the American Locomotive Company (ALCO) and General Electric ...

s, which were high speed passenger locomotives, and the dual service FM Erie-built.

BNSF BNSF Railway is one of the largest freight railroads in North America. One of seven North American Class I railroads, BNSF has 35,000 employees, of track in 28 states, and nearly 8,000 locomotives. It has three transcontinental routes tha ...

took delivery of ES44C4 locomotives with this type of truck in 2009. In the UK, the Class 31 uses this wheel arrangement.

A1A-B+B

"A1A-B+B" means there are three trucks. 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 The SDP45 is a six-axle, C-C, diesel-electric locomotive built by General Motors' Electro-Motive Division of La Grange, Illinois. It was a passenger-hauling version of the SD45 on a stretched locomotive frame with an extended, squared-off lon ...

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, such as the EMD Model 40. This arrangement is sometimes referred to as

0-4-0 Under the Whyte notation for the classification of steam locomotives, represents one of the simplest possible types, that with two axles and four coupled wheels, all of which are driven. The wheels on the earliest four-coupled locomotives were ...

, the

equivalent.

B-1

"B-1" means there are two trucks. 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 The EMD LWT12 was a diesel–electric power car that was built in 1955 by General Motors Electro-Motive Division (EMD), to pull a lightweight passenger trainset. The General Motors Company developed both components under the project name, ' ...

locomotives built by EMD in 1956.

B-2

"B-2" means there are two trucks. 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 RP-210 locomotives built by

Baldwin Baldwin is a Germanic name, composed of the elements ''bald'' "bold" and ''win'' "friend". People * Baldwin (name) Places Canada * Baldwin, York Regional Municipality, Ontario * Baldwin, Ontario, in Sudbury District * Baldwin's Mills, Qu ...

in 1956 and 1957 for use with

Pullman-Standard The Pullman Company, founded by George Pullman, was a manufacturer of railroad cars in the mid-to-late 19th century through the first half of the 20th century, during the boom of railroads in the United States. Through rapid late-19th century ...

Train-X equipment.

B-A1A

"B-A1A" means there are two trucks. 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 some of the FM C-liners (most passenger units) built from 1950 to 1955, and the EMD FL9.

B-B

"B-B" means there are two identical trucks. Each truck has two powered axles, a currently popular configuration used in high-speed, low-weight applications such as intermodal trains and

high-speed rail High-speed rail (HSR) is a type of rail system that runs significantly faster than traditional rail, using an integrated system of specialised rolling stock and dedicated tracks. While there is no single standard that applies worldwide, line ...

, as well as in switching. Examples include the EMD GP (General Purpose), EMD F-units,

EMD SW1500 The EMD SW1500 is a Diesel-electric locomotive intended for switching service and built by General Motors' Electro-Motive Division from 1966 to 1974. The SW1500 replaced the SW1200 in the EMD product line. Many railroads regularly used SW1500 ...

, ''Acela Express'' Power Cars,

Siemens Charger The Siemens Charger is a family of diesel-electric passenger locomotives designed and manufactured by Siemens Mobility for the North American market. There are five variants of the Charger, tailored for different operators and types of servi ...

, Siemens ACS-64 and

GE Genesis General Electric Genesis (officially trademarked GENESIS) is a series of passenger diesel locomotives produced by GE Transportation, then a subsidiary of General Electric. Between 1992 and 2001, a total of 321 units were built for Amtrak, Metro ...

units. High speed ("time") freight trains, with guaranteed schedules often use B-B locomotives of 3,800 HP (950 HP per axle), but this application, too, has largely been replaced by higher-powered, 4,500 HP C-C locomotives (750 HP per axle). An American

Colloquialism Colloquialism (), also called colloquial language, everyday language or general parlance, is the linguistic style used for casual (informal) communication. It is the most common functional style of speech, the idiom normally employed in conversa ...

of "B-B" is "Four axle".

B-2-B

"B-2-B" means there are three trucks. The center truck has two unpowered axles and the truck at each end has two powered axles. The locomotive frame must either articulate or allow for significant side play to be provided to the center truck.

B-B-B

"B-B-B" means there are three trucks. Each truck has two powered axles. The locomotive frame must either articulate or allow for significant side play to be provided to the center truck. The Russian VL 85 and US-American EMD GM10B was a notable example. See also

Bo-Bo-Bo A Bo-Bo-Bo or Bo′Bo′Bo′ ( UIC classification) is a locomotive with three independent two-axle bogies with all axles powered by separate traction motors. In the AAR system, this is simplified to B-B-B. The Bo-Bo-Bo configuration is o ...

B+B+B

"B+B+B" means there are three articulated sets of two powered axles each under the unit. The locomotive frame must allow for significant side play to be provided to the center axle set, as well as allowing for end play for the end sets. The ten Mexican Railway GE boxcab electrics of 1923 are examples of this wheel arrangement.

2-B+B-2

"2-B+B-2" means there are 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 The Pennsylvania Railroad DD1 was a class of boxcab electric locomotives built by the Pennsylvania Railroad. The locomotives were developed as part of the railroad's New York Tunnel Extension, which built the original Pennsylvania Station in Ne ...

and DD2

s used this arrangement.

2-B+B+B+B-2

"2-B+B+B+B-2" means there are 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, hinged to yet another set of two powered axles. Two of these articulated sets are placed back to back and connected by a hinge. Examples include the Milwaukee Road EF-1 "Boxcab" electrics.

B+B-B+B

"B+B-B+B" means there are four trucks under the unit. Within each truck, there are two powered axles, and pairs of them are connected by span bolsters. One example would be the General Electric U50, built from 1963 to 1965. The 4500 horsepower (3.4 MW) turbine locomotives built by GE for

Union Pacific The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pac ...

also used this arrangement. The EFVM railway of

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

uses

narrow gauge A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge narrower than standard . Most narrow-gauge railways are between and . Since narrow-gauge railways are usually built with tighter curves, smaller struc ...

GE "BB" locomotives with this arrangement, both with "standard" and wide cabs. A GE Dash 9-40BBW, for instance, is a wide cab

GE Dash 9-40CW The GE Dash 9-40CW is a diesel-electric locomotive built by GE Transportation Systems of Erie, Pennsylvania, between January 1996 and December 2004. 1,090 were built for Norfolk Southern Railway, as road numbers 8889 to 9978. 53 GE Dash 8-44CWs bu ...

series locomotive with a B+B-B+B wheel arrangement.

B-B+B-B

"B-B+B-B" means that the locomotive has four trucks. 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

's M-10002 diesel streamliner and

New York Central The New York Central Railroad was a railroad primarily operating in the Great Lakes and Mid-Atlantic regions of the United States. The railroad primarily connected greater New York and Boston in the east with Chicago and St. Louis in the Mid ...

's T-Motor third rail electric locomotives are examples 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" means there are four trucks. Each truck has two powered axles. The locomotive frame must allow for significant side play to be provided to the center trucks.

B-B+B-B+B-B

"B-B+B-B+B-B" means that the locomotive has six trucks. 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" means there are 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 (e.g. the PRR B1). This arrangement is sometimes referred to as

0-6-0 Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of no leading wheels, six powered and coupled driving wheels on three axles and no trailing wheels. This was the most common wheel arrangement ...

, the

equivalent.

C-B

"C-B" means there are two trucks. The "C" truck is under the front of the unit, and has three powered axles. The "B" truck is under the rear of the unit, and has two powered axles. The Japanese DE10 and DE11 locomotives use this wheel arrangement.

C-C

"C-C" means there are two identical trucks. Each truck has three powered axles. Examples include the EMD SD (Special Duty), GMD GF6C, EMD GM6C, PRR E44,

GE E60 The GE E60 is a family of six-axle C-C electric locomotives made by GE Transportation Systems (GE) between 1972 and 1983. The E60s were produced in several variants for both freight and passenger use in the United States and Mexico. GE des ...

, Virginian EL-C and GE Evolution Series units, except the ES44C4 and ET44C4 which use the A1A-A1A wheel arrangement. This is a currently popular configuration used in low-speed, high-weight applications, such as unit coal trains. General ("manifest") freight trains also use C-C locomotives. See also Co-Co. An American

colloquialism Colloquialism (), also called colloquial language, everyday language or general parlance, is the linguistic style used for casual (informal) communication. It is the most common functional style of speech, the idiom normally employed in conversa ...

of "C-C" is "Six axle".

1-C+C-1

"1-C+C-1" means there are 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" means there are 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

Pennsylvania Railroad The Pennsylvania Railroad (reporting mark PRR), legal name The Pennsylvania Railroad Company also known as the "Pennsy", was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. It was named ...

's GG1 electric locomotives were notable examples of this arrangement.

2+C-C+2

"2+C-C+2" means there are two sets of axles under the unit. Within each of these sets, there is a guiding truck with two idler axles, and inboard of this, and hinged to it, is a truck with three powered axles. The GE steam turbine-electric locomotives of 1939 were notable examples of this arrangement.

2-C1+2-C1-B

"2-C1+2-C1-B" means there are 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 Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...

-fired steam-turbine locomotives built by the Baldwin Locomotive Works for the

Chesapeake and Ohio Railway The Chesapeake and Ohio Railway was a Class I railroad formed in 1869 in Virginia from several smaller Virginia railroads begun in the 19th century. Led by industrialist Collis P. Huntington, it reached from Virginia's capital city of Richmond ...

between 1947 and 1948. This locomotive is sometimes called the M-1.

C-C+C-C

"C-C+C-C" means there are four trucks 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 General Electric Company (GE) is an American multinational conglomerate founded in 1892, and incorporated in New York state and headquartered in Boston. The company operated in sectors including healthcare, aviation, power, renewable energ ...

for

. These locomotives consisted of two permanently coupled C-C units.

C+C-C+C

"C+C-C+C" means there are four trucks. Each truck has three powered axles and pairs of them are connected by span bolsters. This arrangement was used on the '' Jawn Henry''

-fired steam-turbine locomotive built by the Baldwin Locomotive Works for the

Norfolk & Western Railway The Norfolk and Western Railway , commonly called the N&W, was a US class I railroad, formed by more than 200 railroad mergers between 1838 and 1982. It was headquartered in Roanoke, Virginia, for most of its existence. Its motto was "Precisi ...

in May, 1954.

1-D-1

"1-D-1" means there are three trucks under the unit. At either end are trucks with one idler axle; the center truck has four powered axles. The original 1904–1909

S-Motor third-rail electric locomotives (for the

Grand Central Terminal Grand Central Terminal (GCT; also referred to as Grand Central Station or simply as Grand Central) is a commuter rail terminal located at 42nd Street and Park Avenue in Midtown Manhattan, New York City. Grand Central is the southern termi ...

electrification) and the Great Northern Z-1 electric locomotives (for the Cascade Tunnel electrification) used this arrangement.

2-D-2

"2-D-2" means there are three trucks. At either end are trucks with two idler axles; the center truck has four powered axles. The

PRR R1 PRR may refer to: * Parietal reach region, of the human brain * Pattern recognition receptor, receptors of the innate immune system that identify pathogen-associated molecular patterns * Pennsylvania Railroad (reporting mark), an American railroad ...

electric locomotive used this arrangement.

D-D

"D-D" means there are two trucks. Each truck has four powered axles. Examples include the EMD DD units. D-D locomotives so far have fallen out of favor as nearly all of these have been twin-engined locomotives, which placed too much horsepower in too few axles, thereby making these

consist In rail transport, a train (from Old French , from Latin , "to pull, to draw") is a series of connected vehicles that run along a railway track and transport people or freight. Trains are typically pulled or pushed by locomotives (often kno ...

s rather inflexible (each locomotive featured two

prime mover Prime mover may refer to: Philosophy *Unmoved mover, a concept in Aristotle's writings Engineering * Prime mover (engine), motor, a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc) into energy ...

s, making each unit essentially a pair of high-powered B-B locomotives on a common frame as far as traction and power was concerned). In fact, a usual consist of a D-D unit included a leading C-C unit and a trailing C-C unit, for a total of about 12,600 HP (with four total prime-movers). With today's higher horsepower C-C units (about 4,300 HP apiece), three such C-C units exceeds the total power of the usual D-D consist by 300 HP (with one fewer prime-mover than a usual D-D consist, thereby significantly improving reliability and dramatically reducing maintenance). Although at this time the term D-D is associated with twin-prime-mover locomotives of high power, this does not preclude the possibility of a D-D with a single prime mover of high power some time in the future, nor of the term being used to describe a two-truck, eight-axle electric locomotive. The EMD DDM45 is a narrow-gauge adaptation of the SD45, which required additional axles due to the use of smaller traction motors.

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 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

DR-12-8-1500/2 "Centipede"

diesel locomotive A diesel locomotive is a type of railway locomotive in which the prime mover is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conveyed to the driving whe ...

s and the GE " Little Joe"

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, and inboard of it are four powered axles. Two of these articulated sets are placed back to back and connected by a hinge. The W-1 class of electric locomotives built by General Electric for the Great Northern Railway used this arrangement.

1B+D+D+B1

"1B+D+D+B1" means there are four sets of articulated axles under the unit. At each end, there is one unpowered axle and two powered axles, hinged to a set of 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 The Chicago, Milwaukee, St. Paul and Pacific Railroad (CMStP&P), often referred to as the "Milwaukee Road" , was a Class I railroad that operated in the Midwest and Northwest of the United States from 1847 until 1986. The company experienced ...

used this arrangement.

(B+B-B+B)+(B+B-B+B)

"(B+B-B+B)+(B+B-B+B)" means there are 2 units, each with 4 trucks in a B+B-B+B wheel arrangement. An example was the

Virginian Railway The Virginian Railway was a Class I railroad located in Virginia and West Virginia in the United States. The VGN was created to transport high quality "smokeless" bituminous coal from southern West Virginia to port at Hampton Roads. History ...

's EL-2B electric locomotives.

References

{{DEFAULTSORT:Aar Wheel Arrangement Locomotive classification systems