Under the Whyte notation for the classification of steam locomotives, 0-4-4 represents the wheel arrangement of no leading wheels, four powered and coupled driving wheels on two axles, and four trailing wheels on two axles. This type was only used for tank locomotives.
In the UK 0-4-4 tanks were mainly used for suburban or rural passenger duties. In America, the wheel arrangement became known as the Forney, after a specific design of 0-4-4s, the Forney locomotive, became heavily used on the narrow curves of elevated railways and other rapid transit lines.