This is good. I was just wondering about protocol for these sucky situations. One tip: Don't necessarily wait for signals of distress from subjects of harassment. Many harassees (especially solo ones) will plaster a smile on their faces (to protectively underscore the alleged non-threatening playfulness of the harassment) while speeding up their walking. That is a sign to say something.
Have to share even if no one is still reading this thread (b/c I'm still kind of freaking). Was just riding in the back of a rt. 44, two mid-teens boys were creeping on a woman--sniffing her hair, blowing on her--while the woman kept her eyes closed, headphones on, ignoring. I told them to knock it the fuck off, that they were being creepy. They began denials, then some dude to my right got involved. He totally flipped out on the boys, ended up briefly choking one of them. They practically flew off the bus. So, just sayin'...if options 1, 2, and 3 don't work...