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.
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 particulary difficult when going in reverse. This of course only undermines Theo's confidence and he firmly believes he can not do anything, although this is not entirely true. Theo is quiet and thoughtful, but when he speaks, he is usually direct and 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 is based off of the Brill Tramway's Aveling and Porter engines. He has many modifications including a cab, a set of gears on his right side similar to those of the Coalbrookdale locomotive, and the removal of his footplate.
Theo also has the appearance of an Amberly Chalk Pits 0-4-0. His geared wheels also bear some similarity to those of the narrow gauge traction engine "Samson", a replica of which resides at the Beamish Museum.
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.
- 2017 - Journey Beyond Sodor
He may appear in the twenty-second season.
- Darren Boyd (UK/US)
- Tomoko Murakami (Japan)
- Emiko Takeuchi (Japan; Journey Beyond Sodor trailer only)
- Stefan Pawłowski (Poland)
- Alan Prieto (Latin America)
- Thibault Lacour (France and French speaking Canada)
- Tammo Kaulbarsch (Germany)
- Szabo Mate (Hungary)
- 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, therefore also being the first Thomas & Friends character with a speech impediment.
- Similarly to Fergus, a semicircle has been cut into Theo's front bufferbeam to accommodate a face.
- On the back of the Journey Beyond Sodor DVD, Theo's lamp is white instead of gold.
- Theo is the second traction engine to run on rails, the first being Fergus. However, including non-rail traction engines (not counting George and Buster), Theo is the third traction engine introduced to the series.
- Theo fretting and saying "I think I can't, I think I can't," is very similar, if not a direct reference to the famous children's book "The Little Engine That Could."