I like the composition of the "Greyhound" shot and that I happened to get what I did going 30-40 mph. I find that a lot of my iPhone photography from the metro (particularly the landscapes) is just luck. As for Fort Totten, I like the moodiness of it (and of course, the reflections--I get lots of reflections on the Metro in the evening).
I'm learning as I go. It's not like I can carry my Nikon dslr with me everywhere but I always have my iPhone. So I made the decision to pay attention and photograph scenes that interest me in some way. I'm still waiting to get a specific seat on the train so I can get a specific shot going into a tunnel from outside.
The thing about photographing on the metro, especially at night, is all the interesting reflections and lighting that occurs. Almost all of my shots have the "oh shit" bar in them if you look closely. Almost all of my shots have a reflection of a passenger or two. It can't be helped and sometimes it really adds to the photo.