They really didn’t think that episode through. With how long they were locked in there anyone would’ve either wet themselves (to whatever extent ponies can when they’re not wearing anything) or peed in the room in a way that keeps them dry, as if behind a bush. Yet, we never see any puddles (for obvious reasons), nor do we see them desperate. Do ponies only pee once every few days or something, and Pinkie just got unlucky on the train ride?
Asking the real questions.
Here. https://derpibooru.org/1198782
whers page 15
But Starlight Glimmer locked them in with no bathroom!