A regular stop button would not do anything until the train actually stops at a station, it's really just to give signal to open the door, so I think that might be an actual emergency stop button that the artist drew closer to the girl for convenience sake. But if she pressed that, because it makes the brakes stop the train immediately, it can result in people getting hurt from falling down. And if there is no actual emergency (like fire or madman with a knife), all she would be doing is highly inconveniencing people and whole system of transportation, by making the train be very late. Hence why it's illegal to do it without damn good reason, and no girl is cute enough to get out of it by pretending she panicked over a missed stop.