Wheel arrangement, in the field of railroad locomotives, is how the wheels of the locomotive are arranged by type, position, and connections. There are several notations used to describe wheel arrangements. In the United States and the British Isles, the Whyte notation is generally used for steam locomotives. In Europe, the UIC classification scheme is generally used, especially for diesel and electric locomotives. British practice uses a slightly simplified form of that notation for diesels and electrics. The United States uses the AAR wheel arrangement scheme for diesels and electrics.
Especially in steam days, wheel arrangement was an important attribute of a locomotive, because there were many different ones, each optimised for a different use. Modern diesel and electric locomotives are much more uniform, with all axles driven.