On February 11, 2018, Disney announced that Runaway Railway would be opening in 2019 <ref name="2018blog"> https://disneyparks.disney.go.com/blog/2018/02/d23j-update-mickey-and-minnies-runaway-railway/ </ref>. At this time it was further revealed that guests would begin the attraction by watching a Mickey Mouse cartoon where Mickey and Minnie are getting ready for a picnic. As the duo drives along side Goofy conducting a train, guests would then enter the cartoon. Also showcased at this time, was the fact that a new theme song would be created especially for the attraction <ref name="2018blog"/>
During the development of Mickey and Minnie's Runaway Railway, a new character named "Chuuby" emerged as a star of the attraction. While the singing bird was originally created as a background character, over time he gained a more prominent role ref> https://allears.net/2020/03/02/its-almost-time-to-meet-chuuby-one-of-the-stars-of-mickey-and-minnies-runaway-railway-in-disney-world/ </ref>. Chuuby's name was a result of a misspelling of the word chubby that stuck.
The score for Mickey and Minnie's Runaway Railway was composed by Chris Willis, who also composed music for the current incarnation of Mickey Mouse shorts <ref> https://insidethemagic.net/2018/11/new-details-of-mickey-minnies-runaway-railway-revealed-at-d23s-destination-d/ </ref>. According to Rafferty, the score was recorded in Hollywood, with a 65 piece orchestra.