Could it be a warning for the steps up and down into the train? Or a warning that you should not be using your phone in the carriage?
Phone is close, it's a somewhat misleading image which suggests there are mobile phone repeaters in the carriage. Will you get 5 bars on your signal?!
Well I can say I've noticed no difference on the train. In fact, the data connection on my laptop is as bad as it's ever been for many segments of my journey to my office.
Is it just hot air? A waste of a bunch of stickers? I think so.....we don't need more people making more phone calls on the train either!