The fifteenth season of the television series aired during March 2011 in the UK and between April and June in the US. There are a total of twenty episodes.
- Gordon and Ferdinand - Gordon is embarrassed to be seen with Ferdinand whilst delivering the Lion of Sodor.
- Toby and Bash - Toby tries to find Bash a home on Sodor, but Bash misses his friends on Misty Island.
- Emily and Dash - Emily has to escort Dash to a steam fair, but thoughtlessly leaves him behind.
- Percy's New Friends - After his friends say they are too busy to play, Percy decides to make friends with the animals on Sodor.
- Edward the Hero - Edward wants to be a hero like Harold.
- James to the Rescue - James refuses to work with Toby as the rescue engine, but soon regrets his decision.
- Happy Hiro - Hiro is feeling homesick, so Thomas tries to cheer him up by taking him to Misty Island.
- Up, Up and Away! - Thomas and Percy deliver Mr. Bubbles' giant balloon.
- Henry's Happy Coal - 'Arry and Bert tease Henry about using special coal.
- Let it Snow - Whilst collecting logs for an animal shelter, Thomas sings the Misty Island Snow Song in hopes of making it snow.
- Surprise, Surprise - Thomas tries to hold a Christmas party at the Sodor Steamworks for his friends, but does not tell Victor.
- Spencer the Grand - Spencer disobeys orders and tries to puff to the Duke and Duchess of Boxford's Summer House in the fog.
- Stop that Bus! - Thomas takes Bertie on a tour, but Bertie has to pick up Dowager Hatt and her friends.
- Stuck on You - Thomas and Butch do not listen to Victor's instructions, so they do not know how to work Butch's new magnet.
- Big Belle - Belle's attempts to be friends with Toby only frighten the steam tram away.
- Kevin the Steamie - Kevin practices shunting so that he can take part in Thomas and Percy's game.
- Wonky Whistle - Thomas rushes off before his whistle can be properly fixed and scares Farmer McColl's animals.
- Percy the Snowman - Thomas promises to take a depressed Percy to pick up Sir Topham Hatt, but whilst Thomas is finishing his jobs, he only succeeds in covering Percy in snow.
- Tree Trouble - The Diesels and Thomas compete to find the best Christmas tree for the Steamworks and Dieselworks.
- Fiery Flynn - Flynn tries to impress the Diesels with his bravado with potentially disastrous results.
- Bash and Dash
- 'Arry and Bert
- Sir Topham Hatt
- Lady Hatt
- Dowager Hatt
- Farmer McColl
- The Duke of Boxford
- Mr. Bubbles
- The Sodor Search and Rescue Centre Manager
- The Maithwaite Stationmaster
- The Small Boy
- The Blonde-haired Boy
- The Duchess of Boxford (does not speak)
- Scruff (cameo)
- Annie and Clarabel (cameo)
- Henrietta (cameo)
- Trevor (cameo)
- Stephen Hatt (cameo)
- Bridget Hatt (cameo)
- Mr. Percival (cameo)
- Jem Cole (cameo)
- Alicia Botti (cameo)
- Sodor Brass Band (cameo)
- The Photographer (cameo)
- The Bargeman (cameo)
- The Fireman (cameo)
- The Knapford Stationmaster (cameo)
- The Teacher (cameo)
- The Bird Watcher (cameo)
- The Railway Inspector (cameo)
- The Tree Specialists (cameo)
- Three Sodor United Players (cameo)
- Sir Handel (role cut)
- Peter Sam (role cut)
- Duke (role cut)
- Farmer Trotter (mentioned)
- Father Christmas (mentioned)
- The Railway Coal Inspector
- Norman (not named)
- Dowager Hatt's Friends (do not speak)
- Paxton (cameo)
UK and AUS
- Michael Angelis as the narrator
- Ben Small as Thomas, Toby, and Ferdinand
- Teresa Gallagher as Emily, Rosie, Belle, Mavis, Lady Hatt, and the Ginger-haired Boy
- Keith Wickham as Gordon, Henry, James, Percy, Edward, Whiff, Dash, Salty, Den, Norman, Harold, Captain, Sir Topham Hatt, Dowager Hatt, Mr. Bubbles, and the Railway Coal Inspector
- Matt Wilkinson as Spencer, Stanley, Charlie, Victor, Bash, Cranky, Kevin, Farmer McColl, the Duke of Boxford, the Maithwaite Stationmaster, and the Sodor Search and Rescue Centre Manager
- Rupert Degas as Bertie, Butch, Dart, and Flynn
- Togo Igawa as Hiro
- Kerry Shale as Diesel, 'Arry, and Bert
- Michael Brandon as the narrator, Diesel, and Mr. Bubbles
- Martin Sherman as Thomas and Percy
- Jules de Jongh as Emily, Rosie, Mavis, and Lady Hatt
- William Hope as Toby, Edward, Whiff, Dash, Rocky, the Duke of Boxford, and the Maithwaite Stationmaster
- Glenn Wrage as Spencer, Charlie, Ferdinand, Cranky, Butch, and the Sodor Search and Rescue Centre Manager
- Kerry Shale as Gordon, Henry, James, Bash, Kevin, Harold, 'Arry, Bert, and Sir Topham Hatt
- Keith Wickham as Dowager Hatt, Captain, Salty, and Den
- Ben Small as Stanley
- Teresa Gallagher as Belle and the Ginger-haired Boy
- Rupert Degas as Dart, Flynn, and Bertie
- Togo Igawa as Hiro
- David Bedella as Victor
- This season was supposed to be shown after Day of the Diesels, but was brought forwards for unknown reasons.
- Most of the episodes of this season were written by then head writer, Sharon Miller.
- This was Christopher Skala's last season as executive producer.
- This was Jo Jordan's last season as creative producer.
- This is the last season not to feature the Skarloey Railway engines since their debut, following the eighth, thirteenth, and fourteenth seasons. However, Sir Handel, Peter Sam and Duke appear in the original storyboards of Kevin the Steamie but were cut for unknown reasons. It would have marked their first appearance in CGI, and Duke's first appearance since the fourth season. Sir Handel and Peter Sam were later reintroduced into the sixteenth season.
- Percy's whistle now budges up and down when he uses it.
- This season marks Butch's first speaking role.
- Rupert Degas is now part of the voice cast.
- Jules de Jongh is mentioned in the UK credits despite not having a role.
- On Netflix UK, this series is listed as being season 13.
- This is the only season where Thomas does not get his name titled in any episode.