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

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