The 517 Class were small 0-4-2T tank engines designed by George Armstrong for local passenger work on the Great Western Railway.

The class was far from uniform and encompassed three different wheelbases, saddle and side tanks, and various boilers to name just a few variations.

The independence of the brothers Armstrong is aptly symbolised by the fact that Joseph at Swindon preferred the 2-4-0T wheel arrangement (the 455 Class "Metro" Tanks) to the 0-4-2; George, on the other hand, built no 2-4-0Ts at Wolverhampton.

In the 19th century, the 517s were principally Northern Division engines, and when new worked the Birmingham and Wolverhampton suburban traffic.

[5] When autotrains were introduced on the GWR, a trial was made of enclosing the engine in coachwork to resemble the coaches.