The film tells the story of Lily Stone, the granddaughter of the caretaker of an enchanted steam engine who is lacking an appropriate supply of coal, and Mr. Conductor of Shining Time Station, whose provisions of magical gold dust are at a critical low. To ameliorate these problems, Lily and Mr. Conductor enlist the help of Thomas, who confronts the ruthless and devious Diesel 10 and P.T. Boomer who plan to get rid of the railway's steam engines along the way.
- Alec Baldwin as Mr. Conductor
- Ciarán Hinds as Burnett Stone
- TBA as P.T. Boomer
- TBA as Lily Stone
- TBA as Junior
- TBA as Patch
- TBA as Stacy Jones
- TBA as Billy Twofeathers
- TBA as Sir Topham Hatt
- TBA as "Father"
- TBA as "Son"
- Joseph May voices Thomas
A friendly, blue tank engine who runs his own branch line.
- Kerry Shale voices:
A bumbling olive diesel and Splatter's twin.
A blue tender engine who does the main line express and is the Flying Scotsman's brother.
A green mixed-traffic tender engine who is smaller than Gordon.
A white rescue helicopter who flies around Sodor.
- Rob Rackstraw voices James
A boastful red splendid mixed-traffic engine.
- Christopher Ragland voices Percy
A green saddle tank engine who takes the mail and is Thomas' best friend.
- Jules de Jongh voices Emily
A very kind and friendly tender engine who is always sensitive to the other engines' feelings.
- Yvonne Grundy voices Nia
A Kenyan tank engine who befriended and accompanied Thomas on his journey around the world.
- Rachael Miller voices Rebecca
A large tender engine that originates from The Mainland.
- Eddie Redmayne voices Ryan
A purple tank engine who takes goods on the Harwick Branch Line.
- Teresa Gallagher voices:
A 0-6-0 diesel-mechanical shunting engine.
- Annie and Clarabel
Thomas' branch line coaches.
- Michael Brandon voices Diesel
A diesel shunting locomotive.
- John Hasler voices Rheneas
A narrow gauge tank locomotive who works on the Skarloey Railway.
- Matt Wilkinson voices:
- Diesel 10
An evil Diesel locomotive with a hydraulic claw, who hates steam engines and wants to destroy them.
A bumbling purple diesel and Dodge's twin.
- Diesel 10
- Britt Allcroft voices Lady
A beautiful lost magical tank engine, owned by Burnett Stone who runs the Magic Railroad.
Additionally, Martin Sherman, who originally voiced Thomas, and former Thomas & Friends narrators Michael Angelis and Mark Moraghan are cast in undisclosed voice roles.