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Theo is a shy experimental railway traction engine who works at Ulfstead Castle.


Thomas & Friends

When Thomas travelled to the Mainland, he befriended the experimental engines: Theo, Lexi and Merlin. Theo and Lexi warmly welcomed Thomas and Theo hauled Thomas to refill his coal. Later, Theo, along with the other experimental engines, helped Thomas rescue James, who had been trapped in the Steelworks.

Afterwards, Thomas convinced the experimental engines to stay at the Steelworks with Frankie and Hurricane.


Theo is very shy, extremely cautious and highly awkward. He has an unusual, experimental drive system that does not always function the way it should. He finds it particularly difficult when going in reverse. This only undermines Theo's confidence and he firmly believes he cannot do anything, although this is not entirely true. Theo is quiet and thoughtful, but when he speaks, he is usually direct, to the point and sometimes blunt. He speaks his mind and is usually a pessimist of thoughts, but Theo is kind and generally caring. He would just like to keep to himself and his friends as safe as possible so they can hide away and have a quiet life.


Theo appears to be mainly based on an Aveling & Porter TJ class locomotive, like Fergus, albeit with heavy modifications. Unlike a standard TJ class, Theo has a cab, which may have been inspired by Aveling & Porter's Sir Vincent. He also has an experimental gear drive system on his right side, with a similar setup to that of the Coalbrookdale Locomotive. This system drives both sets of wheels using one large cog, giving him an 0-4-0 arrangement rather than the standard 2-2-0. Because they are both driven by one large cog, his front wheels rotate faster than his rear wheels, causing his gear system to jam frequently. His fly wheel may also have been inspired by Aveling & Porter's No. 807.


Theo is painted greyish brown with gold lining, head lamp and whistle. His wheels and lower traction wheels are dark green while his upper traction wheels are a reddish brown. Theo also carries his name on gold nameplates fixated on each side of his smoke box.


  • It is said that Theo displays personality traits of autism, making him the first Thomas & Friends character with a neurological disability.
  • Theo speaks with a mild lisp.
  • Similarly to Fergus, a semicircle has been cut into Theo's front bufferbeam to accommodate a face.
  • Theo fretting and saying "I think I can't, I think I can't" is a direct reference to the famous children's book The Little Engine That Could.
  • The engine inside Cleo reuses some of Theo's cogs.