- “Not like Merlin... I am Merlin, the experimental prototype King Arthur class stealth engine!”
- ―Merlin introducing himself
When Thomas travelled to the Mainland, he befriended the experimental engines: Merlin, Theo and Lexi. Merlin sails through life, full of confidence, believing that he has the ability to become invisible. Helping to hide Thomas from Frankie and Hurricane, he promised to help Thomas find his way back to Sodor. Later, Merlin, along with the other experimentals, helped Thomas rescue James, who had been trapped in the Steelworks.
Afterwards, Merlin offered to stay at the Steelworks to help Frankie and Hurricane and helped Thomas to convince the other experimental engines Theo and Lexi to do the same.
Merlin sees himself as a hero. He is proud and confident of his abilities and in particular his special skill - the ability to make himself invisible. All he has to do is say "Invisibility on!" and shut his eyes and he is convinced that nobody can see him. He knows he was specially designed in order to be hard to see and his three funnels, which were intended to disperse his smoke and steam, are there to prove it. What Merlin does not know is that his experiment did not work. Merlin wants to be able to prove himself and show everyone all the things he can do. He honestly believes you can do anything you set your mind to or at the very least, you can try! Merlin is fun and funny and wants to be a good friend, but his lack of judgment and sense of his own limitations can create its own challenges.
Merlin is based on the LSWR N15 Class, referred to as the "King Arthur" class by the Southern Railway, specifically No. 783 Sir Gillemere. In 1940, Sir Gillemere was fitted with three stovepipe chimneys by Oliver Bulleid as a wartime experiment. The way the smoke was spread out was meant to make the trains less visible to bombers overhead. The experiment was not a success, and the engine was quickly converted back to normal. No. 783 became No. 30783 under British Railways after nationalisation in 1948 and was withdrawn in March 1961.
Merlin's name is likely taken from another King Arthur locomotive, No. 740 Merlin. A single King Arthur is preserved as part of the National Collection, No. 777 Sir Lamiel, and is based at the Great Central Railway, currently awaiting overhaul.
Merlin is painted silver with a red running board and wheels, and black smoke deflectors. He has red and gold nameplates on either sides. His number is also painted white on his front bufferbeam, the sides of his cab and tender.
- 2017 - Journey Beyond Sodor
He may appear in the twenty-second season.
- Hugh Bonneville (UK/US)
- Kenta Miyake (Japan)
- Grzegorz Pawlak (Poland)
- Miguel Ángel Ghigliazza (Latin America)
- Martin Lohmann (Germany)
- Christian Fokov (Bulgaria)
- Merlin's whistle is Spencer's basis, an LNER A4 Pacific 3-chime.
- Merlin's tender is actually a modified version of Bert's.
- Merlin is the second character to have his basis named in the show, with BoCo being the first.
- According to Behind the Scenes: Thomas & Friends: Journey Beyond Sodor, Merlin's facial features were based off of his voice actor, Hugh Bonneville .
- “Invisibility, on!”
- ―Merlin's catchphrase, Journey Beyond Sodor
- “I'm invisible! Or can't you see? Who said that? You ask. It's only me! You look surprised, but have no fear! It's just a little trick to make myself disappear!”
- ―Merlin's solo in 'We Can't Do Anything'
- “We can't do anything!"
"But we can try! We can give anything a try! We will give anything a try! Isn't that right?”
- ―Merlin responding positively to Lexi and Theo, Journey Beyond Sodor