Sir Haydn is an 0-4-2ST (formerly a 0-4-0) built in 1878 by the Hughes' Loco and Tramway Engine Works Ltd. for use on the Corris Railway.
He worked on the nearby Corris Railway until the closure of that line in 1948. In 1951, he was purchased by the Talyllyn Railway, along with the other surviving Corris locomotive, who became Edward Thomas, and was named after the line's late owner, Sir Henry Haydn Jones. The precarious state of the track meant he was seldomly used for the first few years, and firebox problems caused his withdrawal in 1957. He re-entered service in 1968.
In April 2012, Sir Haydn made his second visit to the Corris Railway and worked there until May 17, 2012 when his boiler certificate expired. However, he remained at the Corris Railway for the rest of the summer as a static exhibit to generate interest in the Corris Railway's plans for their next new-build locomotive; a copy of Sir Haydn. During 2013, Sir Haydn toured railways across Great Britain to raise funds for an overhaul. After this, he returned to the Talyllyn Railway in March 2015, where he had one final public appearance before going in for his overhaul. Sir Haydn was stored (interestingly enough with the "Sir Handel" nameplate decal on one side of his saddletank) in the sheds at Pendre, awaiting a new boiler.
In Autumn 2015, Sir Haydn was transferred from the Talyllyn Railway to the workshops of the Vale of Rheidol Railway in Aberystwyth for his overhaul to be carried out. His overhaul is aimed to be completed in 2018 in time for his 140th anniversary.
Sir Haydn is currently painted in the Corris Railway livery of red with yellow and black lining.
Sir Haydn was mentioned in The Island of Sodor: Its People, History and Railways.
- A book named "Hugh Goes Sliding" written by Christopher Awdry, featured Sir Haydn and Edward Thomas working on the Corris Railway. It was based on a true life event, reminiscent of the Railway Series. It implies that before being purchased by the Talyllyn Railway, Sir Haydn was named "Hugh" after his designer Henry Hughes. Hugh was also featured in a story for the Corris Railway's newsletter, known as the "Corris-Pondent" alongside Cora.
- A derailment involving Sir Haydn on the Talyllyn Railway inspired his Railway Series counterpart Sir Handel to have quite a few similar accidents in both the books and the television series.