Theo is an experimental railway traction engine. Geared engine Theo is very shy, extremely cautious and awkward. He has an unusual experimental drive system that doesn’t 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.”
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.
- He will meet Pooh, Littlefoot, Simba, Sora, Mickey, SpongeBob, Tai, Twilight Sparkle and their friends in Pooh's Adventures of Thomas & Friends: Journey Beyond Sodor.