With a group of teenagers, call 911 or tell the conductor so they can call the police.

If it’s a station where police are stationed then I wish I could tell you to go get the police, but that usually ends with some smarmy comment from the cops about “so why didn’t you do something about it?” They’re more likely to respond through “official” channels.

What you don’t want to do is get involved on your own. If you’re in your 30s or older, the fact is that teenagers are often faster, stronger and have a lot less impulse control than you. They won’t mind beating the shit out of you and running away. If it’s you and a couple of other guys, then maybe you can intervene.