I have recently taken to drawing my fellow train passengers, and I can't believe I didn't do it earlier. It has taught me how draw quickly, as it's often a race before the person walks away to get their train. You never know how long you've got to get the essential lines down on paper.
Everyone is wrapped in layers against the wind and rain, hiding their faces in scarves and their hands in pockets. The details often make the image come alive, for example this lady's shoes and rummaging in her handbag in the waiting room.
It is also great to draw people across the tracks on other platforms, they are so tiny it helps you summarise and just get the general lean and direction of the figure. Compared to life drawing, clothed figures are less anonymous and have more identity, conveyed in their clothes, hair and stance.
The gentleman below was absorbed in reading his newspaper. So far no one has noticed me drawing them, which I was surprised about at first. But people get lost in their phones, books and newspapers.