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)
- Farmer Trotter (mentioned)
- The Railway Coal Inspector
- Norman (not named, does not speak)
- 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, the Duke of Boxford, and the Maithwaite Stationmaster
- Glenn Wrage as Spencer, Charlie, Ferdinand, Rocky, 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 former 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.
- 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.