0-4-0

Under the Whyte notation for the classification of steam locomotives, 0-4-0 represents one of the simplest possible types, that with two axles and four coupled wheels, all of which are driven. In normal circumstances, the wheels on each end of the axles are connected with coupling rods to form a single driven set.

In Britain the Whyte notation of wheel arrangement was also often used for the classification of electric and diesel-electric locomotives with side-rod coupled driving wheels. Other equivalent classifications are:

In the UIC classification used in Europe and, in more recent years, in simplified form in the United States, an 0-4-0 is classified as B (German and Italian) if the axles are connected by side rods or gearing and 020 (French), independent of axle motoring. The UIC's Bo classification indicates that the axles are independently motored, which would be 0-2-2-0 in the Whyte notation.