Choo-Choo

Choo-Choo is the toy train that 2nd Dimension Doofenshmirtz lost in his childhood. Because of its loss, he turned into an evil dictator that ruled the 2nd Dimension Tri-state area. For normal Doofenshmirtz, he never actually lost Choo-Choo but he found one in a box in his pantry labeled VHS Tapes. It is possible that the train is an allusion to "Santa Claus Is Coming To Town" as when 2nd Dimension Doof recieves from 1st Dimension Doof, He says "CHOO CHOO! My heart! I can feel it melting!" similar to the ice wizard in the hit Christmas show did.