The AAR wheel arrangement system is a method of classifying

locomotive A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...

(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 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'', Cr ...

, it is widely used in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

to describe

diesel Diesel may refer to: * Diesel engine, an internal combustion engine where ignition is caused by compression * Diesel fuel, a liquid fuel used in diesel engines * Diesel locomotive, a railway locomotive in which the prime mover is a diesel engine ...

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 (electricity), battery or a supercapacitor. Locomotives with on-board fuelled prime mover (locomotive), ...

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, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

s, which use the

Whyte notation The 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 twenti ...

instead (except

geared steam locomotive A geared steam locomotive is a type of steam locomotive which uses gearing, usually reduction gearing, in the drivetrain, as opposed to the common directly driven design. This gearing is part of the machinery within the locomotive and should not ...

s, which are instead classified by their model and their number of trucks). The AAR system (like UIC) counts axles, unlike Whyte, which counts 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 freight, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construction ...

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), also known as the 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 car ...

diesel multiple unit ( 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 Diesel, 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 ...

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 largest Class 1 railroads in the United States between 1859 and 1996. The Santa Fe was a pioneer in intermodal freight transport; at variou ...

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 short line railroad in the U.S. state of Texas operating between Corpus Christi and the Texas Mexican Railway International Bridge in Laredo, Texas. It is often referred to as the Tex Mex, or Tex Mex Railway. ...

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 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, potentially a multiple unit train, often as the lead vehicle; *a veh ...

s built by

ALCO The American Locomotive Company (often shortened to ALCO, ALCo or Alco) was an American manufacturer that operated from 1901 to 1969, initially specializing in the production of locomotives but later diversifying and fabricating at various time ...

/ ACF in 1935 and 1937 for use with the

Rebel A rebel is a participant in a rebellion. Rebel or rebels may also refer to: People * Rebel (given name) * Rebel (surname) * Patriot (American Revolution), during the American Revolution * American Southerners, as a form of self-identification; ...

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 FM OP800 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 The ''General Pershing Zephyr'' was the ninth of the Chicago, Burlington & Quincy Railroad's Burlington Zephyr, ''Zephyr'' streamliners, and the last built as an integrated streamliner rather than a train hauled by an EMD E-unit diesel locomoti ...

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 al ...

cab unit, only one of which was built for the

Chicago and North Western Railway The Chicago and North Western was a Railroad classes#Class I, 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 t ...

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, Illi ...

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 (G ...

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

BNSF BNSF Railway is the largest freight railroad in the United States. One of six North American Class I railroads, BNSF has 36,000 employees, of track in 28 states, and over 8,000 locomotives. It has three transcontinental routes that provide ...

took delivery of

ES44C4 The Evolution Series is a line of diesel locomotives built by GE Transportation Systems (now owned by Wabtec), initially designed to meet the U.S. EPA's Tier 2 locomotive emissions standards that took effect in 2005. The line is the direct su ...

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.

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. It is also used on speeders. 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 locomotives built by EMD in 1956.

B1-1B

The EMD GT46PAC (WDP-4) Passenger-hauling diesel locomotive of

Indian Railways Indian Railways is a state-owned enterprise that is organised as a departmental undertaking of the Ministry of Railways (India), Ministry of Railways of the Government of India and operates India's national railway system. , it manages the fou ...

, Ten initial units of which were built in 2000-01 by the then GM-owned EMD, initially had this configuration, for better acceleration and speeds, after which every Indian-built WDP-4 had this configuration, until it was developed into "WDP-4B", with a C-C configuration. Twenty SD70ACe-P4 locomotives were built by EMD with a B1-1B arrangement that has two three axle trucks with each truck having two AC

traction motors A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric or hydrogen vehicles, or electric multiple unit trains. Traction motors are used in electrically powered railway vehicles (electric multiple u ...

and one idler axle nearest the fuel tank. The locomotive was designed as an AC traction alternative to the SD70M-2 that uses three DC traction motors on each of two three axle trucks. Only

ordered the model in 2014. The other locomotive with this wheel arrangement is the EMD SD70MACH, which is an SD70MAC rebuilt by

Progress Rail Progress Rail Services Corporation , a fully owned subsidiary of Caterpillar since 2006, is a supplier of railroad and transit system products and services headquartered in Albertville, Alabama. Founded as a recycling company in 1982, Progress Ra ...

for

Metra Metra is the primary commuter rail system in the Chicago metropolitan area serving the city of Chicago and its surrounding suburbs via the Union Pacific Railroad, BNSF Railway, and other railroads. The system operates 243 train station, stati ...

and used for passenger service. In 2020, Metra approved of the purchase of 15 of these locomotives with options to purchase up to 27 more.

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 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 d ...

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 ("dual power"). Sixty units were built ...

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 transport network utilising trains that run significantly faster than those of traditional rail, using an integrated system of specialised rolling stock and dedicated railway track, tracks. While there is ...

, as well as

switcher locomotive A switcher locomotive (American English), shunter locomotive (British English), station pilot (British English), or shifter locomotive (Pennsylvania Railroad terminology) is a locomotive used for maneuvering railway vehicles over short distanc ...

s. Examples include the EMD GP (General Purpose),

EMD F-units EMD F-units are a line of diesel-electric locomotives produced between November 1939 and November 1960 by General Motors Electro-Motive Division and General Motors-Diesel Division. Final assembly for all F-units was at the GM-EMD plant at La Gran ...

EMD SW1500 The EMD SW1500 is a diesel-electric switcher locomotive 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 for road freight service. ...

, ''Acela Express'' Power Cars,

Siemens Charger The Siemens Charger is a family of diesel-electric/ dual-mode 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 ...

Siemens ACS-64 The Siemens ACS-64, or Amtrak Cities Sprinter, is an electric locomotive designed by Siemens Mobility for use on the Northeast Corridor (NEC) and the Keystone Corridor in the northeastern United States. The design was based on locomotives Sieme ...

and

GE Genesis The GE Genesis (officially trademarked GENESIS) is a series of passenger diesel locomotives built by GE Transportation for Amtrak, Metro-North, and Via Rail between 1992 and 2001. A total of 321 units were produced. Designed to meet Amtrak's ...

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'', ''colloquial speech'', ''everyday language'', or ''general parlance'') is the linguistic style used for casual and informal communication. It is the most common form of speech in conversation amo ...

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. Examples of this type were built as

light rail Light rail (or light rail transit, abbreviated to LRT) is a form of passenger urban rail transit that uses rolling stock derived from tram technology National Conference of the Transportation Research Board while also having some features from ...

vehicles.

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

VL85 The VL85 () is a Soviet Union, Soviet (and later Russian) built electric mainline freight locomotive manufactured at the Novocherkassk Electric Locomotive Plant (NEVZ) and designed under the management of V.Ya.Sverdlov (ru:В.Я.Свердлов) ...

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 due to the system only taking powered ...

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 The Mexican Railway (''Ferrocarril Mexicano'') was one of the primary pre-nationalization railways of Mexico. Incorporated in London in September 1864 as the Imperial Mexican Railway (''Ferrocarril Imperial Mexicano'') to complete an earlier p ...

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 New ...

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 is a Class I freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United States after BNSF, ...

also used this arrangement. The EFVM railway of

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

uses

narrow gauge A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge (distance between the rails) narrower than . Most narrow-gauge railways are between and . Since narrow-gauge railways are usually built with Minimum railw ...

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 8-44CW ...

series locomotive with a B+B-B+B wheel arrangement. The EMD SD70ACe-BB produced for

Metre Gauge Metre-gauge railways ( US: meter-gauge railways) are narrow-gauge railways with track gauge of or 1 metre. Metre gauge is used in around of tracks around the world. It was used by several European colonial powers including France, Britain and ...

railway from 2015 onwards, also has a B+B-B+B 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 Midw ...

'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. The WAG-12 Twin-section Electric locomotive of 12,000 HP (9,000 kW), built by

Alstom Alstom SA () is a French multinational rolling stock manufacturer which operates worldwide in rail transport markets. It is active in the fields of passenger transportation, signaling, and locomotives, producing high-speed, suburban, regional ...

since 2017, for the

Broad Gauge A broad-gauge railway is a railway with a track gauge (the distance between the rails) broader than the used by standard-gauge railways. Broad gauge of , more known as Russian gauge, is the dominant track gauge in former Soviet Union countries ...

tracks, also uses the B-B+B-B arrangement.

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 is the Whyte notation designation for steam locomotives with a wheel arrangement of no leading wheels, six powered and coupled driving wheels on three axles, and no trailing wheels. Historically, this was the most common wheel arrangement used o ...

, 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 g ...

, DE11, and DE15 and the

British Rail Class 28 The British Rail Class 28 ( Metro-Vick Type 2) diesel-electric locomotives, known variously as 'Metrovicks', 'Crossleys' or 'Co-Bos', were built under the Pilot Scheme for diesel locomotives as part of the British Railways 1955 Modernisation P ...

are the only locomotives to 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 design ...

, Virginian EL-C and

GE Evolution Series The Evolution Series is a line of diesel locomotives built by GE Transportation Systems (now owned by Wabtec), initially designed to meet the United States, U.S. United States Environmental Protection Agency, EPA's United States emission standa ...

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

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 as 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. At its ...

's GG1 and Companhia Paulista's 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 Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

-fired steam-turbine locomotives built by the

Baldwin Locomotive Works The Baldwin Locomotive Works (BLW) was an American manufacturer of railway locomotives from 1825 to 1951. Originally located in Philadelphia, Pennsylvania, it moved to nearby Eddystone, Pennsylvania, Eddystone in the early 20th century. The com ...

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 Potter Huntington, it reached from Virginia's capital city of Rich ...

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) was an American Multinational corporation, multinational Conglomerate (company), conglomerate founded in 1892, incorporated in the New York (state), state of New York and headquartered in Boston. Over the year ...

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

for the Norfolk & Western Railway 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 station, terminal located at 42nd Street (Manhattan), 42nd Street and Park Avenue in Midtown Manhattan, New York Ci ...

electrification) and the Great Northern Z-1 electric locomotives (for the

Cascade Tunnel The Cascade Tunnel refers to two railroad tunnels, its original tunnel 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 ...

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 The Pennsylvania Railroad's class R1 comprised a single prototype electric locomotive constructed in 1934 by the Baldwin Locomotive Works of Philadelphia, Pennsylvania, US, with the electrical equipment by Westinghouse. It was built as a compet ...

electric locomotive used this arrangement.

D-D

"D-D" means there are two trucks each with four powered axles. Examples include the EMD DD units. D-D locomotives have fallen out of favor as nearly all of these were twin-engined locomotives, which placed too much horsepower in too few axles which made these

consist A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...

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 or motor), a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc.) into ener ...

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 (with four total prime-movers). With today's higher horsepower C-C units (about apiece), three such C-C units exceeds the total power of the usual D-D consist by (with one fewer prime-mover than a usual D-D consist, thereby significantly improving reliability and dramatically reducing maintenance). Although the D-D arrangement is associated with twin-prime-mover locomotives of high power, this does not mean a D-D with a single high-power prime mover may be built in the future, nor does it exclude two-truck, eight-axle electric locomotive. The EMD DDM45 is a narrow-gauge adaptation of the

SD45 The SD45 is a six-axle diesel-electric locomotive class built by General Motors Electro-Motive Division between 1965 and 1971. It has an EMD 645E3 twenty-cylinder engine generating on the same frame as the SD38, SD39, SD40, and SDP40. As of ...

, which required additional axles due to using 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 locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), power source is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is con ...

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), better known as the Milwaukee Road , was a Class I railroad that operated in the Midwestern United States, Midwest and Pacific Northwest, Northwest of the United States from 1847 ...

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 B, or b, is the second Letter (alphabet), letter of the Latin alphabet, used in the English alphabet, modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''English alphabe ...

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