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.
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 based on an Aveling & Porter TJ class locomotive, like Fergus, albeit with heavy modifications. Unlike a standard TJ class, Theo has a cab, as well as 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.
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
- 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.
- 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 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.
- In all merchandise lines, Theo is depicted with only his rear wheel being driven by the cog, therefore having a 2-2-0 arrangement rather than the 0-4-0 depicted in the series. This is to prevent the gears from locking up or breaking, as the front wheels would rotate quicker than the rear wheels if both were connected to the gear. His plarail unmotorized model is the exception, being remedied by both wheels being the exact same size.
- A copy of Theo's flywheel is shown in Crocks Scrap Yard in the twenty-first series episode, P.A. Problems.
- One of Theo's cogs is shown in Brazil in the twenty-third series episode Batucada.