Moderately crowded trains can be good places to observe and sketch people. They tend to stay fairly still and they tend to keep themselves occupied, making it easy to make unobtrusive sketches.
I recently made a number of journeys to London, and these are some of my sketches made on one such (I have omitted the few that include potentially recognisable faces). It was a busy train; I stood all the way from Basingstoke (a little over an hour).
But my children were happy to sit on the floor.