Sir Haydn was built in 1878 by Hughes' Loco and Tramway Engine Works Ltd. of Loughborough as a 0-4-0ST (he is now a 0-4-2ST). No. 3 had been unnamed on the Corris, but received the name Sir Haydn upon arrival on the Talyllyn Railway, after the former owner of the railway; it seems that at that time it was not appreciated the crucial role Haydn Jones had played in keeping the Corris line running in the 1930s, which had played an important part in ensuring the locomotive's survival. Along with No. 4, it kept its Corris number, giving both locos the unusual distinction of carrying the same number though the ownership of four different railway companies (the Corris, Great Western, British Railways and Talyllyn Railway). Because both railways were built to the unusual gauge of 2 ft 3 in (686 mm) it was relatively easy to adapt the Corris locomotives to work on the Talyllyn Railway. Sir Haydn became the first new locomotive to travel on the railway for over 90 years. Upon arriving Sir Haydn was the engine in better condition but frequently derailed however and could not be used safely. Upon inspection it turned out that the Talyllyn track was laid approximately 0.5 inches (13 mm) wider than the official gauge, a deliberate policy by the old company to accommodate the long wheelbase of Talyllyn. The two original Talyllyn locomotives had unusually wide wheel treads that allowed them to stay on the wide-of-gauge track. This problem was eventually cured by the fitting of wider wheel treads and by a Territorial Army members' exercise in 1953 relaying the railway to its correct gauge making Sir Haydn usable. The original cab was replaced in the 1960s.
- Sir Haydn is also Sir Handel's brother.