This is a great site: Underground New York Public Library. I don't know why it is, but it just is if you like books. The legend: 'a visual library featuring the Reading-Riders of the NYC subways'. If you spend a bit of time there, looking at the pictures, seeing what subway travellers are reading, you'll either get why I like it or you won't. It would be good if someone did the same thing for the London Underground.