Drawing on the London Underground is harder than drawing on a train. For a start, people can get off at any time, but they are also facing you, which makes it hard to be subtle. When I was in London recently I had a couple of goes, but mostly when I could see people weren't looking!
I spotted this man as we sat down to rest our feet in the Museum of London. He was too good to miss, but I was sitting in the wrong place, so I had to get John to swap seats with me. Miraculously, he was so absorbed in his new book that even our pantomime didn't draw his attention and he never looked up once.