They will. We're talking of poeple that have no problem stopping a train in a busy station to completely paint it from one side to the other. They'll spray the dog, put a couple stickers on it, and run away.
https://youtu.be/vg5OrytF_a0?t=65
TL;DW: Basically, the conductor alerts the police and stops the train, once they realize that the train is being decorated. If they started moving forward, it could seriously endanger the graffiti sprayers. So that's really the best course of action here.
My first thought after seeing the end of the video was "why is this video flipped?" until I realized that it was only the train that was "upside down".