It'd be difficult to know if this one was actually fooling tourists though. It's a subtle warning of a Penalty Fare if you fail to hide your true emotions fully or make any attempt to engage with passengers. I suppose most tourists would naturally think that we all follow it.
If anything it might make a few groups of loud tourists (usually from across the Pond!) stop reading out all the station names on the line maps in the the Tube and asking everyone how many more stops before Lie-chester Square!