I love drawing with my Japanese ink brush pen. I do a quick pencil sketch and expand it with wet ink line. I realized, it was a flash really, that I don't have to anxiously keep bobbing my head around to keep one person in view to do a portrait. Or gently nudge someone so I can see past them! As people move on and off the subway, or get obscured, I simply look for someone a bit like them at nearly the same angle and keep drawing. I had people on the left and right of me watching eagerly as I discovered this and they too were swiveling around to see who would model an ear or mouth or jacket. So now I've invented the composite portrait. Ta da!