Chuggington (2008)

Season 1: Episode 4

The trainees are heading for a lesson with Dunbar in the training yard when an underground tunnel catches Koko's eye. She wants to explore it but Vee says she isn't ready yet. Dunbar is called away from the lesson about switching tracks before they barely get started. With time on her wheels, Koko decides she's going to see where that tunnel goes.