Thomas is a small blue tank engine. He has come to Sodor in May 1915 as a station pilot at Vicarstown until 1946, at which time he has been transferred to Wellsworth following an incident with some trucks. Later that year, after rescuing James from his crash, he has been given charge of the Knapford-Ffarquhar branch line and two coaches, who he names Annie and Clarabel. He is the main protagonist of the television series.
- Home: Island of Sodor
- Train Built: Steamie
- Appearance: Blue tank engine with 6 wheels, black smokebox, red chassis, black buffers, grey face, yellow portholes and white lamp, red lines around his sides, coal bunker and boiler, wheel arches, yellow No. 1 on both his sides.
- Personality: Cheeky, goofy, funny, kind, friendly, clever, honest, determined, brave, smart, cute, wise, stubborn (sometimes), mischievous (sometimes)
- Goal: To be a really useful engine
- Numbers: 1
- Gender: Male
- Occupation: The Fat Controller's No. 1 engine, Percy's best friend, shunter and branch line engine.
- Alignment: Good
- Allies: Edward the Blue Engine (father), Henry the Green Engine (uncle), Gordon the Big Engine (uncle), James the Red Engine (cousin), Percy the Small Engine (best friend), Toby the Tram Engine, Duck the Great Western Engine, Donald and Douglas the Scottish Twin Engines, Oliver the Great Western Engine, Emily the Beautiful Engine (girlfriend/wife), Murdoch the Strong Engine (uncle), Arthur the LMS Tank Engine (brother), Spencer the Silver Engine (sometimes), Mavis the Quarry Diesel, BoCo the Metropolitan Vickers Diesel, Bill and Ben the Saddle Tank Engine Twins (good friends and pals), Fergus the Railway Traction Engine, Annie and Clarabel, Old Slow Coach, Stanley, Stepney (son), Bertie, Terence, Butch, Trevor the Traction Engine, Caroline, Harold, Skarloey, Rheneas, Sir Handel, Peter Sam, Rusty, Duncan, Duke, Freddie, Mighty Mac, Luke, Bertram, Ivo Hugh, Molly the Yellow Engine (mother), Rosie, Lady (beautiful magical friend/second girlfriend), Belle (beautiful friend), Millie, Stephen, Bash, Dash, and Ferdinand the Logging Locos, Hiro (grandfather), Flynn, Connor, Caitlin, Stafford, Winston the Red Car, Victor, Kevin the Crane, Dennis, Charlie the Purple Engine, Billy, Salty, Porter, Harvey, Elizabeth (sometimes), Cranky (sometimes), Jack, Alfie, Oliver the Excavator, Kelly, Ned, Buster the Steam Roller, Nelson, Paxton, Neville, Captain, Owen, Merrick, Hank, Flora, Pip and Emma, Jinty and Pug, Green Arrow, Iron Duke, Wilbert the Forest Engine, City of Truro
- Enemies: Devious Diesel, 'Arry and Bert, Daisy, Diesel 10, Splatter and Dodge, Stanley (formerly), Spencer the Silver Engine (sometimes), Elizabeth (sometimes), Cranky (sometimes), Bulgy, George, Max and Monty, Den, Dart, Norman, Paxton (only in Day of the Diesels), Sidney, D261, The Horrid Lorries, S.C.Ruffey, Troublesome Trucks, Sailor John, Vinnie, Frieda, Gina, Yong Bao, Sir Topham Hatt (upon being scolded for causing too much trouble).
- Likes: Being a Really Useful Engine, being the Fat Controller's No. 1 engine, being nice to his friends, Annie and Clarabel, his friends, Rosie, people and engines who are friendly to him, being on time, working on his branch line, racing, playing tricks on the other engines, blaming engines and people (sometimes), Ashima, shunting trucks, Spencer being nice to him and his friends, saying that it's not his fault (always), being happy, being angry (sometimes), being rude to other engines and people (sometimes).
- Dislikes: Causing confusion and delay, being late, Diesel 10, people and engines who are rude to him, Devious Diesel, his snowplough (sometimes), danger, Diesel telling him that Sir Topham Hatt will scrap all of the steam engines, Billy calling him a bossy engine, Rosie annoying him, not working on his branch line, being banished from his branch line, other engines pulling Annie and Clarabel (sometimes), Sailor John, Vinnie, Frieda, anyone mad at him, Cranky being cranky, Spencer being rude to him and his friends.
- Powers and abilities: Fire, steam, strength and speed.