- Thomas: The main hero of the story who runs a branch line on the Island of Sodor. When Diesel 10 starts causing trouble on the railway, Thomas is determined to save Lady from him.
- Percy: One of Thomas' three best friends who helps him protect Lady.
- Stanley: One of Thomas' three best friends who helps him protect Lady. He and Thomas had worked with Lady on their old railway.
- Toby: One of Thomas' three best friends who helps him protect Lady. He is like an older brother to Molly.
- Molly: One engine helps protect Lady. She and Lady become friends in the end.
- Diesel 10: A scheming diesel who has returned to Sodor to destroy the lost engine and get rid of steam engines once and for all.
- Alfred: An evil steam engine helping Diesel 10 to destroy Lady and the other steam engines.
- Diesel, Arry, Bert, Splatter and Dodge: Diesel 10's hench-diesels. Diesel, Arry, and Bert are trapped in the garbage yard, and kept an eye on by Whiff. Splatter and Dodge meet their fates near the end and fall off the rails.
- Lady: A magical tank engine who generates the energy that keeps Sodor alive.
- Donald and Douglas
- Bill and Ben (music video non-speaking rolls)
- Harley (non-speaking role)
- Stepney(music video non-speaking role)
- Mr. Andrews' locomotives (mentioned)
- Other railway diesel
- Skarloey (cameo)
- Rheneas (cameo)
- Sir Handel (cameo)
- Peter Sam (cameo)
- Rusty (cameo)
- Duncan (cameo)
- Annie and Clarabel
- Henrietta and Victoria (non-speaking roles)
- Martha and Jennifer (non-speaking roles)
- Cranky (non-speaking role)
- Sir Topham Hatt
- Lady Hatt
- Mr. Howard Andrews
First Appearance of D10, Alfred, Cecil, Splatter, Dodge, and Lady
One and only appearance of Cecil