"I get on a train at the train station."
I heard somewhere that "は" is sort of a marker for "As for . . ." So, in a sentence, if you said 私は猫です, that can either mean "I am a cat" or, if someone asked you and your friend what animal they liked, and they said dog, it could mean "As for me, I like cats". Please correct me if I'm wrong.
That's about right. "Wa" is a topic marker. So, it's basically "on the subject of..."
This is why it's different from "ga." "Wa" accents what follows it, "ga" accents what's before it. "Watashi wa sensei desu." = an answer for someone asking what your job is. "Watashi ga sensei desu." = an answer for someone asking who among the people in the classroom is the teacher.
...As I recall, anyway. I may be flipping those meanings, but I don't think I am.
で would point to something you utilize to do something, including locations, so utilizing you being there to do something. In this case, it's like "i get on the train by the means of/by being in/via the train station". に simply points to the end destination of your action. In this case, that would be the action of "getting on/riding" being connected to the "train". That's my understanding of it at least. Therefore, using に with the train station would imply you're directing an action towards it, which isn't true in this case, since it is like a background location for the whole action to take place in/be the means of, as mentioned before. And で wouldn't fit in the "get on the train" part since the train isn't an intermediary object or "means of" between the action and it's goal, but the goal of the action exactly. Otherwise you'd be saying "i got on (?) via the train", implying the train to be the means of riding something else. I'm no expert at this, so take this with a grain of salt. Was pondering this for a minute after not having my answer accepted too, seems like a talent of it's own to get a grasp of these meanings on the fly.