A typical Class SS (Avonside).
The GWR "Avonside" saddle tank engine is a 0-4-0ST type of steam locomotive. Several different versions and variants were built, and most were purchased for the Great Western Railway (GWR).
Many were also built with added sandboxes, like with GWR 57xx's. They originally were owned by private contractors. Many were scrapped, while others have been preserved.