Talyllyn is the twin of Skarloey. Sir Haydn meanwhile is the twin of Sir Handel. Both Sir Haydn and Talyllyn worked on the Talyllyn Railway while Skarloey and Sir Handel worked on the Skarloey Railway in the Railway Series. He appeared in at least four books from 1959 to 1996 with staging parts in both 1965 and 1985.
In Whitehaven, England, Talyllyn was built by the Flecther and Jennings Company in 1864. He was a 0-4-0ST at first, until the problem with his company's construction was the short wheelbase and long rear overhang which led to his turnover to a 0-4-2ST. The Talyllyn Railway gave him much popularity until his condition was poor by 1945 and putted him aside for the time being. Around 1957 and 1958, Talyllyn was rebuilt by Gibbons Bros. Ltd. but proved problematic as of modifications to rebuild him, resulting with an improved performance for his own status.
He first appeared in The Little Old Engine with a non-speaking role at the end of the book. He doesn't speak again in Very Old Engines based on the final page with two other locomotives of the Talyllyn Railway. Once again, he isn't speaking as seen in Great Little Engines at a station with the passenger train during Sir Handel's visit to the railway. However in New Little Engine, Talyllyn is mentioned in that book featuring the final appearance of the Skarloey Railway.