He asked if there were any more like this. I thought there were. Certainly reminded me of some stickers I saw in "Look Mum No Hands" and a quick(ish) Google search revealed they do indeed have them.
As well as "Danger! Rudeness attracts Tube Mice", there's also Cheer Up Tube stickers for "Frowning on a Friday - Strictly Forbidden", "Fixed Penalty, All Sweeties must be shared & Chocolate" and "Loud music prohibited - Unless you sing along too".