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 bolsters or by connecting individual locomotives via solid drawbars instead of couplers.[1]
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 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 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.[2]
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 OP800800hp (600kW) 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 cars built by ALCO/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 FM OP800800hp (600kW) 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 Chargerpower car 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-6-2-101,000hp (750kW) cab unit, only one of which was built for the Chicago and North Western Railway 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 PAs, which were high speed passenger locomotives, and the dual service FM Erie-built. BNSF 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.
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, the Whyte notation 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
Twenty SD70ACe-P4 locomotives were built by EMD with a B1-1B arrangement that has two three axle trucks with each truck having two ACtraction motors 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 BNSF ordered the model in 2014.[3] The other locomotive with this wheel arrangement is the EMD SD70MACH, which is an SD70MAC rebuilt by Progress Rail for Metra 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
"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-StandardTrain-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, as well as switcher locomotives. Examples include the EMD GP (General Purpose), EMD F-units, EMD SW1500, Acela Express Power Cars, Siemens Charger, Siemens ACS-64 and GE 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). 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. Examples of this type were built as light rail 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 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 DD2electric locomotives 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 also used this arrangement. The EFVM railway of Brazil uses narrow gauge 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 series 4,000hp (3,000kW) locomotive with a B+B-B+B wheel arrangement. The EMD SD70ACe-BB produced from 2015 onward 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 Union Pacific's M-10002 diesel streamliner and New York Central's T-Motorthird-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, the Whyte notation 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, DE11, and DE15 and the British Rail Class 28 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, 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 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'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" 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-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 for Union Pacific. These locomotives consisted of two permanently coupled C-C units.
"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 CentralS-Motor third-rail electric locomotives (for the Grand Central Terminal electrification) and the Great Northern Z-1 electric locomotives (for the Cascade Tunnel electrification) used this arrangement.[citation needed]
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" 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 consists rather inflexible (each locomotive featured two prime 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 (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 BaldwinDR-12-8-1500/2 "Centipede"diesel locomotives and the GE "Little Joe" electric locomotives.
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.
A bogie is a chassis or framework that carries a wheelset, attached to a vehicle—a modular subassembly of wheels and axles. Bogies take various forms in various modes of transport. A bogie may remain normally attached or be quickly detachable. It may include a suspension component within it, or be solid and in turn be suspended ; it may be mounted on a swivel, as traditionally on a railway carriage or locomotive, additionally jointed and sprung, or held in place by other means.
The Baldwin DR-12-8-1500/2 was the Baldwin Locomotive Works' first serious attempt at a production road diesel locomotive. The Baldwin type designation was 'DR-12-8-1500/2,' meaning Diesel Road locomotive, with 12 axles, and two engines of 1,500 horsepower (1,100 kW) each. The trucks were configured in a 2-D+D-2 wheel arrangement. The nickname came from the numerous axles set in a nearly unbroken line, much like the legs of a centipede.
The UIC classification of locomotive axle arrangements, sometimes known as the German classification or German system, describes the wheel arrangement of locomotives, multiple units and trams. It is used in much of the world, notable exceptions being the United Kingdom, which uses a slightly simplified form of UIC, and in North America, where the AAR wheel arrangement system is used to describe diesel and electric locomotives. In North America, the Whyte notation is only used for steam locomotives.
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, Illinois. Production ran from May 1937, to December, 1963. The name E-units refers to the model numbers given to each successive type, which all began with E. The E originally stood for eighteen hundred horsepower, the power of the earliest model, but the letter was kept for later models of higher power.
The EMC EA/EB is an early passenger train-hauling diesel locomotive built from May 16, 1937, to 1938 by Electro-Motive Corporation of La Grange, Illinois for the Baltimore and Ohio Railroad. They were the first model in a long line of passenger diesels of similar design known as EMD E-units. Each locomotive unit developed 1,800 horsepower (1,300 kW) from two 900 hp (670 kW) Winton 201-A diesel engines, driving the wheels through an electric transmission—the generator driven by each engine provided current for traction motors. The locomotives were of A1A-A1A wheel arrangement—two three-axle trucks of which only the outer two axles were powered. Six two-unit 3,600 hp (2,700 kW) locomotives were produced, each consisting of a lead cab-equipped EA A unit and a cabless booster EB B unit. They were numbered 51 through 56; the A units bore the bare number and the B units the number followed by 'X'.
The General Pershing Zephyr was the ninth of the Chicago, Burlington & Quincy Railroad's Zephyr streamliners, and the last built as an integrated streamliner rather than a train hauled by an EMD E-unit diesel locomotive. It was constructed in 1939 with bodywork and passenger cars by Budd Company and diesel engine, electric transmission, power truck, and other locomotive equipment by General Motors Electro-Motive Corporation. Because its intended Kansas City to St Louis route passed near the birthplace and boyhood home of famous World War I General John J. Pershing, the train was named after him. The power car was named Silver Charger, after Pershing's horse Charger, while the passenger cars were named after United States Army badges of rank—Silver Leaf, Silver Eagle, and Silver Star.
The GE U50 was an eight-axle, 5,000 hp (3,700 kW) diesel-electric locomotive built by GE Rail. They were twin-engined locomotives, combining two 2,500 hp (1,900 kW) diesel engines.
The SDP45 is a six-axle, C-C, 3,600-horsepower (2,680 kW) 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 long hood at the rear, aft of the radiators, giving space for a steam generator for passenger train heating. This steam generator placement followed the pattern set by the SDP35 and SDP40.
The EMD DD35 was a 5,000 horsepower (3,700 kW) diesel-electric locomotive of D-D wheel arrangement built by General Motors Electro-Motive Division for the Union Pacific Railroad and Southern Pacific Railroad.
The ALCO Century 855 was a model of 5,500 hp (4,101 kW) diesel-electric locomotive built in 1964 by the American Locomotive Company for the Union Pacific. The locomotive was notable for being ALCO's most powerful diesel-electric locomotive and, at the time, the most powerful diesel locomotive ever built, being surpassed by the 6,600-horsepower (4,922-kilowatt) EMD DDA40X in April 1969.
The Evolution Series is a line of diesel locomotives built by GE Transportation Systems, initially designed to meet the U.S. EPA's Tier 2 locomotive emissions standards that took effect in 2005. The line is the direct succesor to the GE Dash 9 Series. The first pre-production units were built in 2003. Evolution Series locomotives are equipped with either AC or DC traction motors, depending on the customer's preference. All are powered by the GE GEVO engine.
The Consolidation Line was a series of diesel-electric railway locomotive designs produced by Fairbanks-Morse and its Canadian licensee, the Canadian Locomotive Company. Railfans have dubbed these locomotives “C-liners”, however F-M referred to the models collectively as the C-Line. A combined total of 165 units were produced by F-M and the CLC between 1950 and 1955.
The EMD DDM45 is a meter-gauge diesel-electric locomotive built by EMD. The DD in the name means that it has eight axles in two trucks, giving it a D-D wheel arrangement, while the M stands for the meter-gauge track it was to operate on, and the 45 represents the EMD SD45 that the engine was derived from. They were built for service on the Estrada de Ferro Vitória a Minas (EFVM) in Brazil. In the early 1970s, the EFVM needed not only to expand its locomotive roster, but to use the most powerful single-engined locomotives available, and the SD45 represented the highest-rated model that EMD had to offer at the time.
Co-Co is the wheel arrangement for diesel and electric locomotives with two six-wheeled bogies with all axles powered, with a separate traction motor per axle. The equivalent UIC classification (Europe) for this arrangement is Co′Co′, or C-C for AAR.
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 manufacture large road locomotives in their own plant at Beloit, Wisconsin, and was concerned that waiting to develop the necessary infrastructure would cause them to miss out on the market opportunity for large road locomotives. Engineering and assembly work was subcontracted out to General Electric, which produced the locomotives at its Erie, Pennsylvania, facility, thereby giving rise to the name "Erie-built."
NSB Di 3 is a class of 35 diesel-electric locomotives built by NOHAB for the Norwegian State Railways (NSB). The class was built between 1954 and 1969, and delivered in two series, Di 3a and Di 3b. They are based on the Electro-Motive Division F7 and are equipped with EMD 567 engines. They have a distinct bulldog nose and were numbered 602–633 (a-series) and 641–643 (b-series). The locomotives had a prime mover that gives a power output of 1,305 kilowatts (1,750 hp). The a-series has a Co′Co′ wheel arrangement, while the b-series has (A1A)(A1A). The b-series has higher top speed, but lower tractive effort.
The Union Pacific GTELs were a series of gas turbine-electric locomotives built by Alco-GE and General Electric between 1952-1961 and operated by Union Pacific from 1952 to 1970.
T41 was a type of diesel-electric locomotive used by Swedish State Railways between 1956 and 1988 for secondary line services and later for shunting. It was built by Nydquist & Holm (NOHAB), who produced five locomotives under licence from General Motors Electro-Motive Division.
The EMD G18 was an export locomotive introduced by GM-EMD in the late 1960s. The standard EMD suffixes applied after the G18 designation to indicate if the customer purchased locomotives with specific traction motors to fit narrow gauge (U) or broad gauge (W) rails. An (L) indicates a locomotive built with a lightweight frame, and (6) indicates A1A-A1A trucks. The similar EMD GA18 was a G18 designed as an extremely light locomotive with low axle loading and used freight car trucks driven by a cardan shaft and underframe mounted traction motors like its predecessor, the EMD GA8. A further variation was the G18B. They are powered by an EMD 8-645E prime mover rated at 1100 bhp and 1000 hp for traction and were produced with A1A-A1A or B-B trucks.
Rigid-framed electric locomotives were some of the first generations of electric locomotive design. When these began the traction motors of these early locomotives, particularly with AC motors, were too large and heavy to be mounted directly to the axles and so were carried on the frame. One of the initial simplest wheel arrangements for a mainline electric locomotive, from around 1900, was the 1′C1′ arrangement, in UIC classification.
References
↑ Pinkepank, Jerry A. (1973). The second diesel spotter's guide. Milwaukee, WI: Kalmbach Publishing. ISBN0-89024-026-4.
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