There is a bit of a thing among Berlin drawing folk, urban sketchers and life drawing people and so on, of “Unterweg” drawings.
They’re the drawings you make on the U-Bahn, S-Bahn, bus or tram, as you’re underway.
I find that I’m using these opportunities to draw regular people going about their day as a place to practise techniques. I can drop on a half-finished drawing and decide to add dark values, or detail, or a second person, in the most relaxed and experimental way.
You get as far as you get with an unterweg, then the person you’re drawing gets off the train, or you do, or they notice you drawing and you have to stop. Often children gather to watch, and adults murmur, “Schon!”