We touched on the social taboo aspects in an earlier thread on street pick ups. I think we are in agreement that it can be a difficult scenario, I tend to stick with what I am best equipped to be successful with, you advocate pushing the envelope.
Nothing wrong with that as long as you pull it off without being a jerk, in my opinion.
It's not really "pushing the envelope" so much as a difference in mentality. First, even on my daily morning commute, at the exact same time every morning, I only ever see a couple of the same people. Meaning even if I embarrass myself or the girl, we'll likely never see each other again. I couldn't care less about what people think about me, especially when I'm never going to see them again.
Secondly though (and this more to the point), one school of thought is "It's not socially appropriate, therefore I won't do it" vs "It's not socially appropriate, so let me find a way to do it appropriately." Think about it, what situations are appropriate for you to talk to someone else on the train? Asking for directions? Telling them they dropped something? Telling them that they're hair is on fire? Then what situations do you not even think about "am I inconveniencing this person?" Maybe when you're trying to read a message from something important and you can't read a Kanji, so you ask someone for help? Recently I started doing a lot of shopping for car parts on Yahoo auction, and those bastards seem like they work hard to make sure I can't understand some of the words they use, so I ask someone around me:
"Excuse me, can you tell me how to read this Kanji? *points to 落札者*"
"Oh sure, it's らくさつしゃ."
"Man, I've been living here for 7 years and Kanji still trips me up!"
*hook* *continue conversation* *change topic*
Stupid conversations like that wouldn't set off alarms to people around us, which avoids the social taboo-ism she might be uncomfortable with. People are more likely to do things against social norms when they think that no one is looking. So don't give people a reason to look.
I'm actually quite impressed with anyone's "train game" if it's subway or city rail (ie. not Shinkansen). I find "on the train" the least hospitable part of Tokyo.
Having recently just experienced true rush hour transit mayhem.. The truth and rumours about Japanese politeness seem to go right out the window with respect to being on trains. It's a friggin war zone. No politeness, just cold elbows in my back and stepping on my nice shoes! Women were pushing me hard! Men were pushing the women! No one let the old man sit! It was anarchy! Chaos! Real "Lord of the Flies" going on there..
"Woman, if you're gonna rub your body against me, at least have the courtesy to buy me dinner first. How about tonight at 7?" =P