- “I think I can't, I think I can't, I know I can't...”
- ―Theo fretting, Journey Beyond Sodor
Theo is a shy experimental railway traction engine.
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 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 appears to be a hybrid between the Coalbrookdale locomotive, an early steam engine built by Richard Trevithick, and the Aveling and Porter locomotives, specifically the 2-2-0WT 'TJ' class railway traction engine No. 9449 "The Blue Circle" (Fergus' basis).
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.
- 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.