Or maybe it's just that I'm very relaxed on the subway. There's no need to control anything when I'm riding a train, so my normally tense brain can relax and allow flashes of insight to come through more easily. That would fit with the fact that the rest of my inspiration has come while sitting at a bar with a drink in hand. When there's no decisions to be made, my brain is at peace enough to think shit up. Like dreaming. If that's the case, maybe there's some way I can meditate my way to that state without depending on the situation of where I happen to be seated.
Or maybe there's no rhyme or reason to it and I'm just assigning significance were none exists.