You are doing beautifully by yourself.

However another source of guidance might be to examine the work of Walker Evans. He traveled the subways of New York 1938-1941.
“EVANS, Walker, "The Passengers", New York, 1938, Walker Evans, Princeton, The Metropolitan Museum of Art, Princeton University Press, 2000.”
http://phomul.canalblog.com/archives...2/1217571.html

Other books that have examples of this work but are not limited to his subway photos:
Walker Evans; Museum Of Modern Art, NY, NY; Szarkowski, John, 1971
Walker Evans; Hambourg, Rosenheim, Eklund, Fineman; Metropolitan Museum of Art, Princeton University Press, Princeton, NJ, 2000

John Powers