The feature-length special has been named the Best Seller in Children's overall by the online retailer, just a day before its DVD hit UK stores.
It fended off other orders such as Despicable Me 2, Monsters University, The Little Mermaid Collection and The Smurfs 2.
Formally 18th, the release also peaked in 11th place on the Best Sellers in Film & TV list, beating the likes of hollywood blockbusters, Pacific Rim, Fast & Furious 6, The World's End, Iron Man 3 on Blu-ray 3D and Man of Steel.
King enjoyed a successful extended run in Vue cinemas with a VIP premiere in Leicester Square and peaked at No. 4 on the cinema chain's Top 10 Films last week.
King of the Railway: The Movie is available to buy on UK DVD.
Thanks to Hawk01 for the alert.