The 33-year-old, who won Best Actor for his role as Stephen Hawking in The Theory of Everything, will star as the voice of Ryan, an engine in the Thomas And Friends special - entitled Sodor's Legend of The Lost Treasure.
Eton educated Eddie said: "I grew up loving to watch Thomas and his pals getting caught up in unexpected and mischievous adventures.
"I jumped at the opportunity to get involved and what fun it was.
"I have never voiced an animated character before, and I had a complete blast."
The cast includes veteran actor John Hurt and Twilight star Jamie Campbell Bower.
The hour-long film will be released in the autumn and is billed as 'an epic story of friendship, betrayal and, ultimately, loyalty where Thomas is at the centre of the action'.
Eddie, whose performance as the scientist also won him a Golden Globe, a Bafta and a Screen Actors Guild award, is also likely to be recognised at the Empire Awards.
He has been nominated for the best actor award and faces competition from two of the men he beat to the best actor Oscar - Benedict Cumberbatch and Bradley Cooper, as well as The Hobbit's Richard Armitage and Andy Serkis from Dawn Of The Planet Of The Apes.
In other Eddie news, he's also trying to lose two stone for upcoming movie The Danish Girl in which he will play a transgender artist Lili Elbe, one of the first men to have surgery to become a woman in 1930.
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