Theo

Theo is a shy experimental railway traction engine who works at the Steelworks on the Mainland.

Biography
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.

Personality
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.

According to executive producer Christopher Keenan, Theo is very particular. He has a great knowledge of mechanics and likes to understand the world around him and how things work. He is open and honest with his emotions, but is not as comfortable with others as he is with mechanics and has trouble with communicating and understanding.

Basis
Theo appears to be mainly based on an Aveling & Porter TJ class locomotive, like Fergus, albeit with heavy modifications. Unlike a standard TJ class, Theo has a cab, which may have been inspired by Aveling & Porter's Sir Vincent. He also has 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. His fly wheel may also have been inspired by Aveling & Porter's No. 807.

Livery
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.

Appearances
Television Series= {{Scroll box|

Thomas & Friends

 * Series 22 - Seeing is Believing
 * Series 24 - Thomas and the Royal Engine

Specials

 * 2017 - Journey Beyond Sodor

Music Videos

 * Journey Beyond Sodor - We Can't Do Anything and The Most Important Thing is Being Friends
 * Series 21 - Set Friendship in Motion

Videos
}} Other Media=
 * 2020 - Top 5 Silliest Characters

Books

 * 2017 - Friends to the Rescue, Journey Beyond Sodor, Journey Beyond Sodor: Sticker Activity Book, Journey Beyond Sodor: The Movie Storybook and Thomas at the Steelworks
 * 2018 - Character Encyclopedia
 * 2020 - Thomas the Tank Engine Encyclopedia

Trivia

 * Theo was created with the intention of displaying personality traits of autism, though was not overtly recognised as such by Mattel.
 * This makes Theo the franchise's first character to be written with the intent of showing neurodivergent traits, the second being Bruno from Thomas & Friends: All Engines Go, who is officially recognised as being its first autistic character.
 * 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.
 * Some parts from Theo's model have previously been reused as set dressing:
 * 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 reused as a part on a flatbed in Brazil in the twenty-third series episode, Batucada.
 * Cleo's engine reuses some of Theo's cogs.
 * Theo is the only Adventures model not to feature a front hook despite having one in the television series.
 * According to Michael White, Theo may have been intended to appear in the twenty-fourth series double length episode, Thomas and the Royal Engine, but was dropped for unknown reasons.

Merchandise
Adventures Plarail Capsule Plarail