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 c ...

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, 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 g ...

s (including third-rail electric locomotives). It is not used for steam locomotives, 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 or wheel assemblies. A plus sign ("+") refers to articulation, either by connecting bogies with

span bolster Span may refer to: Science, technology and engineering * Span (unit), the width of a human hand * Span (engineering), a section between two intermediate supports * Wingspan, the distance between the wingtips of a bird or aircraft * Sorbitan ester ...

s 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 adop ...

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 Electro-Motive Corporation (later Electro-Motive Division, General Motors) produced five 1800 hp B-B experimental passenger train-hauling diesel locomotives in 1935; two company-owned demonstrators, #511 and #512, the Baltimore and Ohio Rai ...

locomotives owned by the Santa Fe Railroad 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 The Texas Mexican Railway was a railroad that operated as a subsidiary of the Kansas City Southern Railway in Texas. It is often referred to as the Tex-Mex, or TexMex Railway. On January 1, 2005, Kansas City Southern took control of the Texas ...

.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 The St. Louis Car Company was a major United States manufacturer of railroad passenger cars, streetcars, interurbans, trolleybuses and locomotives that existed from 1887 to 1974, based in St. Louis, Missouri. History 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 locomo ...

/ 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

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 Baldwin Locomotive Works produced several different Baldwin DR-6 models of 6-axle passenger train-hauling diesel locomotives between 1945 and 1948. The series comprised eight individual versions, all of which sold only in small numbers; across ...

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 befor ...

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 EMD E-units were a line of passenger train streamliner diesel locomotives built by the General Motors Electro-Motive Division (EMD) and its predecessor the Electro-Motive Corporation (EMC). Final assembly for all E-units was in La Grange, Illino ...

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 (GE ...

s, which were high speed passenger locomotives, and the dual service

FM Erie-built The Erie-built was the first streamlined, cab-equipped dual service diesel locomotive built by Fairbanks-Morse, introduced as direct competition to such models as the ALCO PA and FA and EMD FT. F-M lacked the space and staff to design and ma ...

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 that ...

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

under the rear of the unit. The only example to date of this arrangement was a single experimental EMD SDP45.

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, 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 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 in 1956 and 1957 for use with Pullman-Standard 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 The EMD FL9 (New Haven Class EDER-5) is a model of electro-diesel locomotive, capable of operating either as a traditional diesel-electric locomotive or as an electric locomotive powered from a third rail. Sixty units were built between October 1 ...

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, 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 SW1500s f ...

, ''Acela Express'' Power Cars, Siemens Charger, 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 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.

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 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

s. 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 structu ...

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 ...

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

. In most cases, the locomotive is articulated over the span bolster. The

M-10002 The Union Pacific Railroad's M-10002 was a diesel-electric streamliner train built in 1936 by Pullman-Standard, with prime movers from the Winton Engine division of General Motors Corporation and General Electric generator, control equipment an ...

diesel streamliner and New York Central'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 The EMD FT is a diesel-electric locomotive that was produced between March 1939 and November 1945, by General Motors' Electro-Motive Corporation (EMC), later known as GM Electro-Motive Division (EMD). The "F" stood for Fourteen Hundred (1400) ...

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, 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 The is a class of Japanese C-B wheel arrangement diesel-hydraulic locomotives. 708 locomotives were built between 1966 and 1978. , 138 locomotives remained in operation. Variants DE10-0 subclass 158 DE10-0 locomotives were built with steam ...

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 designe ...

Virginian EL-C The Virginian EL-C, later known as the New Haven EF-4 and E33, was an electric locomotive built for the Virginian Railway by General Electric in August 1955. They were the first successful production locomotives to use Ignitron (mercury arc) rect ...

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 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 The Pennsylvania Railroad's class FF1 was an American electric locomotive, a prototype numbered #3931 and nicknamed "Big Liz". It was built in 1917 to haul freight trains across the Allegheny Mountains where the PRR planned to electrify. "Big ...

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'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 rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...

-fired steam-turbine locomotives built by the Baldwin Locomotive Works for the Chesapeake and Ohio Railway 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 en ...

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

s. 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 "Precisio ...

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 New York Central S-Motor third-rail electric locomotives (for the Grand Central Terminal electrification) and the Great Northern Z-1 electric locomotives (for the

Cascade Tunnel The Cascade Tunnel refers to two railroad tunnels (original and its replacement) in the northwest United States, east of the Seattle metropolitan area in the Cascade Range of Washington, at Stevens Pass. It is approximately east of Everett, wit ...

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 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 know ...

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 Baldwin DR-12-8-1500/2 "Centipede" diesel locomotives 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 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's EL-2B electric locomotives.

References

{{DEFAULTSORT:Aar Wheel Arrangement Locomotive classification systems