To a gutless individual like me trains provide the perfect opportunity to sketch people. This is because I get all embarrassed when people realize that I've been sketching them and train travel allows me to deploy two sneaky tactics.
The first tactic is drawing people on the platform. It's like being in a movable bird hide from which to study the many and varied species of general public. I was particularly chuffed when the triangular looking guy bellow pulled in to view in an adjacent train. He inspired me to quickly dig out my sketch pad and jot him down before he pulled away. At the next station I was presented with lady staring in to space in the second drawing, yays!
Mobile phone guy at the same station