This is mainly aimed at guys, but if there's any women out there doing this, you're included.
You're on the train, you forgot to bring along things to keep you occupied, and you're bored stiff. Because of this you can't seem to ignore the booger lodged up your left nostril. You try but it is a powerful thing and compels you to get it. It's okay. It happens to the best of us. Get it. Don't, however, forget this fifteen minutes later when you're still bored and you look at your fingernails and think "huh, I really should trim those," because when your fingertips go in your mouth and the biting begins, well that just crosses a line.
Either one is less than ideal when in public, but we're all human. It happens. But combining the two, specifically in that order? That's a no-go, as we used to say in the army. Just sayin'. People are watching. ;-)
Otherwise, happy commuting!