- "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 travels to the Mainland, he befriends the experimental engines: Theo, Lexi, and Merlin. Theo and Lexi warmly welcome Thomas and Theo hauls Thomas to refill his coal. Later, Theo, along with the other experimental engines, help Thomas rescue James, who had been trapped in the Steelworks.
Afterwards, Thomas convinces the experimentals to stay at the Steelworks with Frankie and Hurricane.
Geared-engine Theo is very shy, extremely cautious, and awkward. He has an unusual experimental drive system that does not always run very smoothly and he makes sudden jolts forward or back when his rods or cogs jam. Quiet and thoughtful, with a habit of blunt speaking, his confidence is often jolted. But Theo is genuinely kind and caring and would just like to keep 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 - the 2-2-0WT 'TJ' class railway traction engine No. 9449 "The Blue Circle" (Fergus' basis), him having a cab could be inspired by another Aveling and Porter locomotive, the 0-4-0WT No. 8800 "Sir Vincent".
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 Season 22.
- 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.