Railway locomotives come in a wide variety of shapes and sizes.  In December 1900, Frederick Methvan Whyte, a mechanical engineer with the New York Central Railroad, came up with a system to classify the various types of locomotives according to their wheel arrangement.  Known as Whyte Notation, the system separates locomotives by noting the number of “pilot wheels”, “driving wheels” and “trailing wheels”.  In addition to Whyte’s system, railroad workers gave ‘nicknames’ to many of the different locomotive classes as well.  

The Whyte Notation does not normally count the wheels under the locomotive’s ‘tender’ or ‘coal car’ – but if you see a “T” in the notation, such as 0-4-0T, that indicates the locomotive is a “tank engine”.  On a tank engine, the water tank is mounted on top of the boiler, rather than on its own tender car hooked on behind the engine.  Geared locomotive, such as Heisler, Shay and Climax, don’t quite fit into the conventional Whyte Notation and usually have their own specific designations.