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TheAAR wheel arrangement system is a method of classifyinglocomotive (or unit)wheel arrangements that was developed by theAssociation of American Railroads. Essentially a simplification of the EuropeanUIC classification,[citation needed][ambiguous] it is widely used inNorth America to describediesel andelectric locomotives (includingthird-rail electric locomotives). It is not used forsteam locomotives anywhere, which use theWhyte notation instead, exceptgeared steam locomotives,[citation needed] which are instead classified by their model and their number of trucks (bogies inCommonwealth English). The AAR system (like UIC) counts axles, unlike Whyte notation, which counts wheels.
Letters refer topowered axles, with 'A' representing one powered axle, 'B' two consecutive powered axles on the same truck (bogie), 'C' three consecutive powered axles on the same truck, and so on.[1]
Numbers are used to show unpowered (idler) axles. "1" refers to one idler axle, and "2" to two idler axles in a row on the one truck (bogie), and so on.[1]
A dash ("–") separatestrucks (bogies) or wheel assemblies. A plus sign ("+") refers to articulation, either by connecting bogies withspan bolsters or by connecting individual locomotives via solid drawbars instead ofcouplers, or inmultiple units, where each unit is permanently coupled to the other.[1]
| Arrangement | Explanation | Example(s) |
|---|---|---|
| 1A-A1 | 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. | Budd RDC diesel multiple unit (DMU) cars. |
| 1B-1B | There are two trucks with a leading idler axle in front of two powered axles. | Used to upgrade the B-B arrangement of twoEMC 1800 hp B-B locomotives owned by theSanta Fe Railroad in 1938, for greater stability at speed. |
| 1-D | 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 a2-8-0 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 theTexas Mexican Railway.[2] |
| 2-A1A | 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. | TheFM OP800 800 hp (600 kW) railcar, six of which were built by theSt. Louis Car Company exclusively for theSouthern Railway in 1939. |
| 2-B | 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. | The three lightweightpower cars built byALCO/ACF in 1935 and 1937 for use with theRebel streamliners. |
| 3-A1A | 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. | The later built of theFM OP800 800 hp (600 kW) railcar, six of which were built by theSt. Louis Car Company exclusively for theSouthern Railway in 1939. |
| A1-1A | 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 | 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. | 'Silver Charger'power car for theGeneral Pershing Zephyr. |
| A1A-3 | 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. | TheBaldwin DR-6-2-10 1,000 hp (750 kW) cab unit, only one of which was built for theChicago and North Western Railway in 1948. |
| A1A-A1A | There are two trucks under the locomotive. Each truck has two powered axles, with anidler 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. | TheEMD E-units andALCO PAs, which were high speed passenger locomotives, and the dual serviceFM Erie-built.BNSF took delivery ofES44C4 locomotives with this type of truck in 2009 and theET44C4 in 2015. In the UK, theClass 31 uses this wheel arrangement. TheWabtec FLXDrive also uses this arrangement. |
| A1A-B+B | 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 aspan bolster under the rear of the unit. | The only example to date[when?] of this arrangement was a single experimentalEMD SDP45. |
| B | 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 theEMD Model 40. It is also used onspeeders. This arrangement is sometimes referred to as0-4-0, theWhyte notation equivalent. |
| B-1 | 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. | The threeEMD LWT12 locomotives built byEMD in 1956. |
| B1-1B | In a unit with this arrangement, there are two trucks: one of which only has one idler axle at the back of the unit, and the other has two consecutive powered axles facing the front, and another idler axle, which faces the back. | TheEMD GT46PAC (WDP-4) passenger-hauling diesel locomotive ofIndian Railways, Ten initial units of which were built in 2000-01 by the then GM-ownedEMD, 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. TwentySD70ACe-P4 locomotives were built byEMD with a B1-1B arrangement that has two three axle trucks with each truck having twoACtraction motors and one idler axle nearest the fuel tank. The locomotive was designed as an AC traction alternative to theSD70M-2 that uses threeDC traction motors on each of two three axle trucks. OnlyBNSF ordered the model in 2014.[3] The other locomotive with this wheel arrangement is the EMDSD70MACH, which is anSD70MAC rebuilt byProgress Rail forMetra 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.[4] |
| B-2 | 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. | The three lightweightRP-210 locomotives built byBaldwin in 1956 and 1957 for use withPullman-StandardTrain-X equipment. |
| B-A1A | 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. | Some of theFM C-liners (most passenger units) built from 1950 to 1955, and theEMD FL9. |
| B-B | There are two identical trucks, each of which has two powered axles. An Americancolloquialism of "B-B" is "Four axle". | B-B is a currently popular configuration used in high-speed, low-weight applications such as intermodal trains andhigh-speed rail, as well asswitcher locomotives. Other examples are theEMD GP (General Purpose),EMD F-units,EMD SW1500,Acela Express Power Cars,Siemens Charger,Siemens ACS-64 andGE Genesis 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). |
| 1-B-B-1 | A wheel arrangement with pair of swivelling, unpowered wheels on each end flanking two powered bogies, this is known as2-4-4-2 or2-4-0 + 0-4-2 in theWhyte notation. | TheNew Haven EP-1 electric locomotives were rebuilt from a B-B design to this configuration for better stability at speed, becoming the only examples of this wheel arrangement. |
| B-2-B | 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 aslight rail vehicles. Also, theJapanese National Railways'DD51 diesel-hydraulic locomotives uses this arrangement. |
| B-B-B | There are three trucks, each with two powered axles. The locomotive frame must either articulate or allow for significant side play to be provided to the center truck. | The RussianVL85 and AmericanEMD GM10B was a notable example. See alsoBo-Bo-Bo. |
| B+B+B | 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 tenMexican Railway GE boxcab electrics of 1923. |
| 2-B+B-2 | 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. | ThePRR DD1 andDD2electric locomotives used this arrangement. |
| 2-B+B+B+B-2 | 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 theMilwaukee Road EF-1 "Boxcab" electrics. |
| B+B-B+B | There are four trucks under the unit. Within each truck, there are two powered axles, and pairs of them are connected byspan bolsters. | TheGeneral Electric U50, built from 1963 to 1965. The 4500 horsepower (3.4 MW) turbine locomotives built byGE forUnion Pacific also used this arrangement. TheEFVM railway ofBrazil usesnarrow gauge GE "BB" locomotives with this arrangement, both with "standard" and wide cabs. AGE Dash 9-40BBW, for instance, is a wide cabGE Dash 9-40CW series 4,000 hp (3,000 kW) locomotive with a B+B-B+B wheel arrangement. TheEMD SD70ACe-BB produced forBrazil'sMetre Gauge railway from 2015 onwards, also has a B+B-B+B arrangement. |
| B-B+B-B | The unit has four trucks. Each truck contains two powered axles. The middle pair of trucks are connected by aspan bolster. In most cases, the locomotive is articulated over the span bolster. | TheUnion Pacific'sM-10002 diesel streamliner andNew York Central'sT-Motorthird-rail electric locomotives are of this type. This arrangement also includes locomotives made of two permanently coupled B-B units, such as someEMD FT units which had a solid drawbar connecting two units instead of the typical couplers. TheWAG-12 Twin-section Electric locomotive of 12,000 HP (9,000 kW), built byAlstom since 2017, for theIndian Railways'Broad Gauge tracks, also uses the B-B+B-B arrangement. |
| B-B-B-B | 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 | The only known locomotives to have this configuration were the twoEMD TR3 locomotives made of three permanently coupled B-B units, which had solid drawbars connecting the units instead of the typical couplers. | |
| C | 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. thePRR B1). This arrangement is sometimes referred to as0-6-0, theWhyte notation equivalent. |
| C-B | 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 JapaneseDE10,DE11, andDE15 and theBritish Rail Class 28 are the only locomotives to use this wheel arrangement. |
| C-C | There are two identical trucks. Each truck has three powered axles. An Americancolloquialism of "C-C" is "Six axle". | TheEMD SD (Special Duty),GMD GF6C,EMD GM6C,PRR E44,GE E60,Virginian EL-C andGE 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 alsoCo-Co. |
| 1-C+C-1 | 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. | ThePRR FF1 andFF2 electric locomotives used this arrangement. |
| 2-C+C-2 | 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. | ThePennsylvania Railroad'sGG1 andCompanhia Paulista's electric locomotives were notable examples of this arrangement. |
| 2+C-C+2 | 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. | TheGE steam turbine-electric locomotives of 1939. |
| C1+2-C1-B | 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. | See also:Chesapeake and Ohio class M1 The only examples of this arrangement were three uniquecoal-firedsteam-turbine locomotives built by theBaldwin Locomotive Works for theChesapeake and Ohio Railway between 1947 and 1948. This locomotive is sometimes called the M-1. |
| C-C+C-C | 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 byGeneral Electric forUnion Pacific. These locomotives consisted of two permanently coupled C-C units. |
| C+C-C+C | There are four trucks. Each truck has three powered axles and pairs of them are connected byspan bolsters. | This arrangement was used on theJawn Henrycoal-firedsteam-turbine locomotive built by theBaldwin Locomotive Works for theNorfolk & Western Railway in May, 1954. |
| 1-D-1 | 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 1904–1909New York CentralS-Motor third-rail electric locomotives (for theGrand Central Terminal electrification) and theGreat Northern Z-1 electric locomotives (for theCascade Tunnel electrification) used this arrangement.[citation needed] |
| 2-D-2 | There are three trucks. At either end are trucks with two idler axles; the center truck has four powered axles. | ThePRR R1 electric locomotive used this arrangement. |
| D-D | There are two trucks each with four powered axles. 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 theseconsists rather inflexible (each locomotive featured twoprime movers, 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 (9.4 MW) (with four total prime-movers).With today's higher horsepower C-C units (about 4,300 hp (3.2 MW) apiece), three such C-C units exceeds the total power of the usual D-D consist by 300 hp (220 kW) (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. TheEMD DDM45 is a narrow-gauge adaptation of theSD45, which required additional axles due to using smaller traction motors. | See also:Category:D-D locomotives Examples include theEMD DD units,Union Pacific 6936, andUnion Pacific 6916 |
| 2-D+D-2 | 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. | TheBaldwinDR-12-8-1500/2 "Centipede"diesel locomotives and theGE "Little Joe"electric locomotives. |
| B-D+D-B | 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. | TheW-1 class of electric locomotives built by General Electric for theGreat Northern Railway used this arrangement. |
| 1B+D+D+B1 | 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 theMilwaukee Road used this arrangement. |
| (B+B-B+B)+(B+B-B+B) | There are 2 units, each with 4 trucks in aB+B-B+B wheel arrangement. | An example was theVirginian Railway'sEL-2B electric locomotives. |
