I speak three languages and I'm learning a fourth. Don't study grammar, ever, it's a waste of time. Grammar rules always fall into one of two categories: the ones that are so obvious that you would have learned them after two examples anyway or the ones that are too vague and complicated to be useful.
For an example of the latter look up people making flowcharts for the subjunctive or for the ga/wa distinction.
Or, for that matter, find me the place in an english grammar that explains why you get "on" a train, but "in" a car.
For me, it's been helpful to understand grammar in German and I don't consider it a waste of time. Your experience seems to be different and I'm glad you enjoy it that way.
I couldn't tell you why this is a rule, but at least to my Canadian English speaking ears, if you said "Joe walked into a train", I'd think that Joe walked across some train tracks and was struck by the train, not that he boarded it.
Because you "walk into" things and get hurt. Like walls, or trees. The notable exception is buildings, or less tangible things like a spotlight.
Trains, ships, and busses are a weird kind of middle ground between on/in.
Thinking about it, I believe the principal difference is if there is something significant (more significant than the roof of a train) above your head? It seems to be very subjective and though, any time I think I've gotten it figured out another example/edge case comes out.
