K+C Johnson Ltd
WHEEL ARRANGEMENT CLASSIFICATION
STEAM LOCOMOTIVES
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 LOCOMOTIVES
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¯¯¯OOO¯ 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.
GEARED LOCOMOTIVES
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!
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Classification
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