This locomotive was originally named Sextus and was built in 1896 to a gauge of 2' for the Groby Granite Quarry in Leicestershire. By 1909, the locomotive was in need of extensive attention and returned to Hunslet in Leeds. However, the owners reconsidered and decided instead to part-exchange Sextus for a new 0-6-0 locomotive.
In 1910, the vast Dinorwic Quarries near Llanberis needed another locomotive and Hunslet were able to offer Sextus from stock. The locomotive was modified to meet Dinorwic's requirements, including removal of the simple cab, a slight reduction in the gauge to 1'11½", altering the draw gear and fitting a second injector.
The locomotive was renamed Lady Madcap either in 1911 or possibly two years later. This name is believed to be from a horse that won the Hurst Park Welter Flat Race on 31st December 1912. An alternative explanation may be the inspiration was from the eponymous 1904 musical by Paul Rubens, starring Lily Elsie.
The locomotive is similar to the Penrhyn 'Port' class locomotives, and worked at Dinorwic until 1931. Then it received a major overhaul and was transferred up to Allt Ddu to work on the tramroad connecting quarry operations in the Allt Ddu area with the steam mills. Possibly the locomotive had a shorter boiler and water tank from 1931, having been made more like the 'Alice' class locomotives. It continued in service through its next major overhaul in 1947 until withdrawal in the Spring of 1952, to be replaced by Velinheli .
Lady Madcap was mostly broken up, but many parts survived, being collected gradually over a period of years by the new owners. Now restoration is in progress. New frames have been cut and assembled. The four axleboxes have been machined and assembled in the hornguides. The whole frame work has been painted.