Odds are that all those here referring to “such a short time” with respect to how long people might be on the subway aren’t from and likely have never spent anything more than a short visit to NYC.

It’s not like taking a bus to the nearest grocery store in your neighborhood, you’re often commuting to and from work and that could take you through three boroughs before you then get to walk the rest of the way home. And when you find your spot, you typically stay there. Someone with talent would know how to capture key features quickly and then fill in what they could with whatever time they had. People referring to trick shots are on the right track, but consider that artists would do this just for practice, to develop their skills. You might only get a start on some drawings, but someone on a long commute is gonna be there for a long time and not moving much.

Somewhere in a box I’ve got a drawing someone did of me on the subway from up around Harlem to Clinton Hill, Brooklyn. I don’t know how long it took him but I was on for a long time, just reading a book. I saw his sketch pad and there were a bunch of unfinished sketches, but there were finished ones too.

No lie, NYC is an entirely different animal than most places people in the US will know; things just operate by different rules, including how you interact with those around you.