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.”
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