Under the Whyte notation for the classification of steam locomotives, 4-2-0 represents the wheel arrangement of four leading wheels on two axles, two powered and coupled driving wheels on one axle, and no trailing wheels. This type of locomotive, often called a Jervis type, was common on American railroads from the 1830s through the 1850s.
Other equivalent classifications are:
UIC classification: 2′A (also known as German classification and Italian classification)
French classification: 210
Turkish classification: 13
Swiss classification: 1/3
Read more about 4-2-0: History