Steam locomotives are classified by their wheel configurations. On steam locomotives, the wheels are appropriately called drivers. Locomotives may also have sets of non-powered pilot wheels before and/or after the drivers.
Steam locomotives are classified using a system called Whyte Notation developed by Frederick Methvan Whyte, which has been in use since the early 20th century. In Whyte notation, 2-8-4 means that the locomotive has two pilot wheels in front of eight driving wheels, followed by four more trailing wheels. Note that this is very different than the AAR Wheel Arrangement Notation used in classifying diesel and electric locomotives.
Classes were given names frequently. For example, a 2-8-4 locomotive was known as a Berkshire. The Polar Express was pulled by a Berkshire locomotive.
Below are lists of steam locomotive class names, listed by their wheel configurations. The wheel arrangements not listed were primarily less common types, and their class names were usually alphanumeric codes, which could vary from one railroad to another. That is if they were even used by more than one railroad.
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Single-Axle Lead Pilot Truck Classes
These steam locomotives had two leading wheels on one axle. Often, they were in a leading truck.
- 2-2-2 (8-foot drivers): Great Western
- 2-2-2 (7-foot drivers): Star or Firefly
- 2-4-0: Hawthorn or Victoria
- 2-4-2: Columbia
- 2-6-0: Mogul
- 2-6-2: Prairie
- 2-8-0: Consolidation
- 2-8-2: Mikado or Calumet (changed to MacArthur after the attack on Pearl Harbor)
- 2-8-4: Berkshire (Kanawha on the Chesapeake and Ohio Railroad)
- 2-10-0: Decapod
- 2-10-2: Santa Fe
- 2-10-4: Texas
- 2-12-0: Centipede
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Two-Axle Lead Pilot Truck Classes
These classes of steam locomotives had four leading wheels. This provided the stability needed for high-speed operation.
- 4-2-0: Jervis
- 4-2-2: Iron Duke/Rover
- 4-4-0: American
- 4-4-2: Atlantic
- 4-6-0: 10 Wheel
- 4-6-2: Pacific or St. Paul
- 4-6-4: Hudson or Baltic
- 4-8-0: 12 Wheel
- 4-8-2: Mountain
- 4-8-4: Class names listed separately below
- 4-10-0: Mastodon
- 4-10-2: Southern Pacific (named for the primary railroad to use them)
- 4-12-2: Union Pacific (named for the only railroad to use them)
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4-8-4 Locomotive Class Names
On the Union Pacific, among other railroads, 4-8-4 locomotives were the Northern class. Railroads in the Southern states wouldn't call a locomotive on their roster a Northern. This resulted in many alternative names for 4-8-4 locomotives.
- Northern: Union Pacific and various other railroads
- Big Apple: Central of Georgia RR
- Confederation: Canadian National Railway
- Confederation: Grand Trunk Western RR
- Dixie: Nashville, Chattanooga and St. Louis Railway
- General, Governor, or Statesman: Richmond, Fredericksburg, and Potomac RR
- Greenbrier: Chesapeake and Ohio Railway
- GS, Golden State, or General Service: Southern Pacific RR
- Niagara: New York Central RR
- Niagara: Ferrocarriles Nacionales de Mexico
- Pocono: Delaware, Lackawanna and Western RR
- Potomac: Western Maryland Railway
- Western: Denver and Rio Grande Western RR
- Wyoming: Lehigh Valley RR
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Duplex Steam Locomotives
Duplex locomotives had two engines and two sets of drivers on a single rigid frame.
Continue to 5 of 5 below.
- 6-4-4-6: Pennsylvania (the only one made was owned by PRR)
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Articulated Steam Locomotives
Articulated locomotives had multiple engines and sets of drivers that could operate independently of each other.
- 2-6-6-6: Allegheny (C&O) or Blue Ridge (Virginian Railway)
- 2-8-8-0: Bull Moose
- 2-8-8-2: Y Class (Norfolk and Western)
- 2-8-8-4: Yellowstone
- 2-10-10-2: Virginian
- 4-6-6-4: Challenger
- 4-8-8-4: Big Boy
- 2-8-8-8-0: Triplex (1 made)
- 2-8-8-8-2: Triplex (3 made)
- 2-8-8-8-4: Triplex (1 made)