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K+C Johnson Ltd



Here's a handy reference for you. Frederick Methvan Whyte's system of classification is used to describe the wheel arrangement of conventional steam locomotives. In this system, the first number is the number of leading wheels, and the last is the number of trailing wheels. The middle number (or numbers) give the number and arrangement of drivers. A "T" added at the end indicates a tank engine. Note that some railroads had their own particular "type" name.

Whyte Symbol Wheels Type
0-4-0 OO 4-wheel switcher, 4-coupled
0-6-0 OOO 6-wheel switcher, 6-coupled
0-8-0 OOOO 8-wheel switcher, 8-coupled
0-8-8-0 OOOO OOOO Mallet (Articulated)
0-10-0 OOOOO 10-wheel switcher, 10-coupled
0-10-2 OOOOOo Union (for Union RR, Pittsburgh)
2-2-0 oO "Planet"
2-2-2 oOo "Single"
2-4-2 oOOo Columbia/Columbian (for New Zealand)
2-6-0 oOOO Mogul
2-6-2 oOOOo Prairie
2-6-4 oOOOoo Adriatic (quite rare, none in U.S.)
0-6-6-0 OOO OOO Mallet (Compound)
2-6-6-2 oOOO OOOo Mallet (Articulated)
2-6-6-4 oOOO OOOoo Mallet (Articulated)
2-6-6-6 oOOO OOOooo Allegheny ("Blue Ridge" on the Virginian)
2-8-0 oOOOO Consolidation
2-8-2 oOOOOo Mikado (the "Mike"; called "MacArthur"s during WW2!)
2-8-4 oOOOOoo Berkshire (also called "Kanawha" on the C&O, "Lima" on the B&M)
2-8-8-0 oOOOO OOOO Angus, Mallet (Articulated)
2-8-8-2 oOOOO OOOOo Mallet (Articulated), (Cab-Forward on SP, "Chesapeake" on the C&O)
2-8-8-4 oOOOO OOOOoo Yellowstone
2-8-8-8-2 oOOOO OOOO OOOOo Triple Articulated, Triplex
2-8-8-8-4 oOOOO OOOO OOOOoo Triple Articulated, Triplex (rear drivers under tender!)
2-10-0 oOOOOO Decapod
2-10-2 oOOOOOo Santa Fe (known as "Decapod" on the S.P., "Central" on the IC)
2-10-4 oOOOOOoo Texas ("Colorado" on the Burlington!, "Selkirk" on the C.P.)
2-10-10-2 oOOOOO OOOOOo Mallet (Articulated), Virginian
4-2-0 ooO 6-Wheeler, Jervis (Ca 1832!)
4-2-2 or 4-2-4 ooOo or ooOoo Bicycle (some uncertainty here!, more research needed!)
4-4-0 ooOO American, or Eight-Wheeler
4-4-2 ooOOo Atlantic ("Chautauqua" by the Brooks Loco Works, "Milwaukee" on, you guessed it, the Milwaukee Road!
4-4-4 ooOOoo Reading (called "Jubilee" on the Central Pacific)
4-4-4-4 ooOO OOoo 4-cylinder Non-Articulated ("Duplex" on the Pennsy, "B&O" on B"O
4-4-6-2 ooOO OOOo (two built for AT&SF)
4-4-6-4 ooOO OOOoo 4-cylinder Non-Articulated, "Duplex-Drive" for the Pennsy
4-6-0 ooOOO 10-Wheeler
4-6-2 ooOOOo Pacific
4-6-4 ooOOOoo Hudson ("Baltic" on the Milwaukee, "Shoreline" on the New Haven)
4-6-4-4 ooOOO OOo 4-cylinder Non-Articulated
4-6-6 ooOOOooo Tank
4-6-6-2 ooOOO OOOo Cab-Forward (S.P., 1911), 4-cylinder Articulated
4-6-6-4 ooOOO OOOoo Challenger (U.P., 1936), 4-cylinder Articulated
4-8-0 ooOOOO 12-Wheeler, or "Mastodon" (C.P., 1882)
4-8-2 ooOOOOo Mountain ("Mohawk" on the NYC)
4-8-4 ooOOOOoo Northern (also known by a bunch of other names! Confederation, Dixie, Golden State, etc.,etc.)
4-8-8-2 ooOOOO OOOOo Cab-Forward on S.P., 1928, Mallet (Articulated)
4-8-8-4 ooOOOO OOOOoo Big Boy, Mallet (Articulated)
4-10-0 ooOOOOO Mastodon (C.P., 1883)
4-10-2 ooOOOOOo Southern Pacific ("Overland" on U.P.)
4-12-2 ooOOOOOOo Union Pacific
6-2-0 oooO Crampton (Camden & Amboy, 1849)
6-4-4-6 oooOO OOooo Pennsylvania (PRR, 1939)


Diesel wheel arrangements (in the U.S.A.) are classified by the AAR system. One powered axle per truck is indicated by the letter "A", two powered axles by the letter "B", three by the letter "C", and four by the letter "D". Non-powered, or "idler" axles are indicated by numbers. I.e., A1A-A1A describes a locomotive with two trucks, each truck having a powered axle, an idler axle, and another powered axle.

    Examples (red indicates idler axles):
  • ¯OOO¯¯¯OO   A1A-A1A (two trucks, each truck having a powered axle, an idler axle, and another powered axle)
  • ¯OO¯¯¯OO¯   B-B (two powered axles on each of two trucks)
  • ¯OOO¯¯¯OOO¯   C-C (three powered axles on each of two trucks)

I understand that about 90% of all U.S. diesel locomotive have one of the three arrangements above. Don't confuse wheel arrangements with UNIT arrangements such as A-A, A-B, A-B-A.


Shay, Heisler, Climax and Willamette are various types of steam locomotives in which power was transmitted to the drivers via a geared mechanism. The locomotives rode on two or three sets of trucks (four trucks on some Shays). More on these later!


I'm a-workin´ on it!

Updated 02-11-2006, Copyright ©2006 K+C Johnson Ltd