- “Yes Sir, there is. Another engine came to be mended too, called Talyllyn. When the workmen saw us together, they laughed and called us their 'Little Old Twins'.”
- ―Skarloey, The Little Old Engine
Talyllyn is Skarloey's twin and his real life counterpart.
Talyllyn was built alongside his twin Skarloey in 1864 by Fletcher, Jennings & Co. at the Lowca Engineering Works. Skarloey was sent to the Skarloey Railway on Sodor, while Talyllyn went to the Talyllyn Railway in Wales. Two years later, he was joined by Dolgoch.
Originally built in 1864 by Fletcher, Jennings & Co. of Whitehaven as an 0-4-0ST, Talyllyn had a short wheelbase and long rear overhang which led to his rapid conversion to a Class C 0-4-2ST. As the more popular of the Talyllyn Railway's two original locos, Talyllyn was in very poor condition by 1945 when he was laid aside. He was rebuilt in 1957-58 by Gibbons Bros. Ltd., but proved problematic and has undergone considerable modification since then, resulting in a much improved performance.
In April 2014, Talyllyn's right-hand side connecting rod broke, which resulted into him being sent to the Ffestiniog Railway for the connecting rod to be mended, and a bottom-end overhaul to be carried out at the Boston Lodge Works. In August of that year, Talyllyn returned to service, and on 24th of September, he celebrated his 150th birthday in grand style alongside Dolgoch.
In his appearances in the Railway Series, Talyllyn is painted in the Talyllyn Railway's Bronze Green livery with black and yellow lining, and brass fittings. He has red and brass name, number and builder's plates on the sides of his cab.
Talyllyn is currently painted Indian Red with black and gold lining, red wheels and "TAL-Y-LLYN" written in light yellow on the sides of his water tank.
- In the audiobook, The Railway Stories: The Little Old Engine and other stories, Talyllyn is pronounced as "Taly Lyn".
- His name means "The Tall Lake" in Welsh.
- Talyllyn was built at Fletcher, Jennings & Co. at Whitehaven, the same place as Skarloey, Rheneas, Captain Baxter, and Dolgoch.
- Talyllyn is the only steam engine on the railway not to be modified to look like their railway series counterpart at some point.
- In the Railway Series, Talyllyn has never been depicted with a face. However, Japanese box-artwork for the Railway Series depicts Talyllyn with a face. The face resembles one of Thomas' illustrated by C. Reginald Dalby.