I'd be surprised if they were drunk teenagers and more likely they were adults that looked young (this happens a lot with Americans/Europeans mistaking Japanese adults for non-adults).
Even drunk adults on trains is hugely frowned upon.
Ive seen my share of drunks....but aside from coomuters actively avoiding them...no cops ever got involved.
In my experience, its fairly common to see an avg salaryman drunk after work drinks. Usually the tell is a very red blush and borderline falling asleep or just acting drunk, but not loud.
That being said, i dont think i saw teenagers. Or groups of drunks.
Are you saying in cases of group of drunks people, cops got involved?
And as other comments have said, almost any train late at night will have drunk adults as a pretty significant percentage of the passengers.