Subway announcements, although annoying, needn't be unbearable; mind the gap, for instance, is civilizing without being scolding. Pithy without being silly.
But if I have to hear courtesy is contagious, and it starts with you one more time, I will put my umbrella in someone's eye. See how far courtesy gets you
Or I would, if the fine fellows at the 6th precinct hadn't ever so gently suggested I leave it by the door in a bag.