The wheel arrangement method of classifying steam engines is called the Whyte System, named for its creator. The numbers represent the number of wheels on the loco from front to back. At a minimum there should be 3 numbers. The first is the lead or pony truck. The second is the driving wheels which actually move the loco. The third number is the number of wheels under the cab called the trailing truck.
Various engine types also had nick names which were indicative of the railroad which first used that type. Here is a list of some engines with the Whyte system classification, a profile view of the wheels and the nick name of the type as listed in the NMRA DATA SHEET D9a.1
| CLASS | WHEEL VIEW | NAME |
| 2-4-2 | o OO o | Columbia |
| 4-4-0 | oo OO | American |
| 4-6-4 | oo OOO oo | Hudson |
| 4-4-2 | oo OO o | Atlantic |
| 4-6-2 | oo OOO o | Pacific |
| 2-6-2 | o OOO o | Prairie |
| 2-8-0 | o OOOO | Consolidation |
| 2-8-2 | oo OOOO oo | Mikado or Mac Arthur |
| 2-8-4 | o OOOO oo | Berkshire |
| 2-10-4 | o OOOOO oo | Texas |
| 4-8-2 | oo OOOO o | Mountain |
| 4-8-4 | oo OOOO oo | Northern |
| The following locomotives are
really two engines on one frame and share one boiler. They are generally called articulateds. |
||
| 2-10-10-2 | o OOOOO OOOOO o | Virginian |
| 4-4-4-4 | oo OO OO oo | Duplex |
| 4-6-6-4 | oo OOO OOO oo | Challenger |
| 4-8-8-4 | oo OOOO OOOO oo | Big Boy |
Wheel profile key: O=driving - o=pony/trail wheels
The type of locomotives a railroad designed and built depended upon a number of factors:
Freight or Passenger service
Speed required for service
Weight distribution for bridges
Ruling grade of railroad. (that is the steepest hill on the line)
2004 by Joseph D. Korman