Art on the Underground

I was at Baker Street station, getting on to a south-bound Jubilee line train. I got in the carriage and turned round to face back to the platform. And there, on the wall, was a very large picture, a blow up of a pen and ink drawing, it seemed, of a man brandishing a riding crop or a switch or something, standing next to a woman who looked like she was gagged! I nearly fell over! Unfortunately at that point someone else got into the carriage and blocked my view, before I could figure out what was going on in the picture.
Baker Street is famous for being the home of Sherlock Holmes, and the underground station does have a lot of references to the fictional detective, so it's not beyond the realms of possibility that the drawing was an illustration from one of the stories, but what I want to know is how many other semi-kinky pictures are there on the London Underground system?! Trying to spot some will certainly make future journeys more interesting.