Lady Madcap is a Hunslet saddle tank formerly used at Dinorwic State Quarries, now undergoing reconstruction at the WHHR.
The original locomotive (Hunslet 652 of 1896) was built for the Groby Granite Co in Leicestershire where it was named Sextus. In 1909 it was returned to Hunslet for an overhaul, but when it was finished the Groby company decided to trade it in for a new locomotive. Hunslet sold it in 1910 to Dinorwic, becoming that quarry's only Hunslet not bought new. In design it differed from the other Dinorwic Hunslets, being more like the Penrhyn 'Port' class, but it worked until 1952 and was gradually dismantled during the 1950s, and the frames were eventually cut up in 1969-70.
More recently a number of parts including the cylinders have been collected at Gelert's Farm for the reassembly of the locomotive. New frames have been manufactured, and some say that makes it technically a new locomotive, but many genuinely old engines have had new frames over the years without undermining their authenticity (for example the England engines on the FR).
See the WHRL site for more information.
Thomas, C. (2001). Quarry Hunslets of North Wales, Oakwood Press