Duck is a fictional standard gauge tank locomotive created by the Rev. W. Awdry. Duck lives and works on the North Western Railway. He carries the number 8.

Duck first appeared in the second series episode Duck Takes Charge, which first aired in 1986. His last appearance was in the twenty-fourth series episode Thomas and the Inventor's Workshop, which was released in 2021.

Since his introduction, Duck appeared as one of the main core characters up until the seventh series. During the development of the Steam Team, Duck was originally considered to be the eighth member, but was left out to make room for Emily to give a female character a more prominent role. As a result of the stories from the eighth to sixteenth series focusing primarily on the Steam Team, Duck was widely neglected and left absent along with many other characters for several years.