I want to say something, but what?

A small group of loud girls, three or four of them, board the MAX and tromp through the car up to the front raised section, where I’m sitting. It’s the coldest day of winter and one of them’s wearing ultra mini-shorts, her brown legs bare all the way down to the sockless feet in her equally weather-wrong pumps. She shouts to the others “Dude tole me to get some pants on!” “What dude?” asks another. She gestures to the back with her head, her face angry. “That blonde nigger.” The others twist their heads back to see. “He better know who he talking to before he try talking to ME like that! He got NO IDEA who he talking to, and he better watch his ass! Because I got ME some BIIIIIIG PEOPLE! And if he don’t want to be one o’the people that gets SMASHED IIIIIN, he better shut his big ass mouth and he better not talk to ME like THAT!" Her friends emit random sounds of agreement throughout the tirade. Two of them, fully clothed, sit across the aisle from Miss Nudie-legs, each occupying a pair of seats in the sparsely populated section. They sit their behinds on the window seat and pull up their knees to rest their heels on the aisle seat, displaying colorful canvas sneakers of identical make, the kind with inch-high white-walls bordering the soles. The shoes look brand new, except for some dirt that has gotten onto the white parts. The girls begin to rip off pieces from a wad of paper they share, which they then lick and use to rub the white parts of the shoes. After a few rubs they lick the paper again, then more rubbing. When a piece of paper becomes too soiled, they throw it onto the floor, rip off another one, lick it, and start again. The white walls are getting cleaner, but the seat upholstery underneath the girls' feet is accumulating layers of dirt. Soon both of the seats are covered with the dirt from the shoes, to the point where no one would intentionally sit there. I suspect it does not occur to them that in their endeavor to be clean in one area (their shoes), they are being piggies in several other areas. I consider pointing it out to them. But I've just gotten off a long day of work, I'm tired, and I can't think of a way of phrasing this that will not result in my being pelted by industrial-strength verbal abuse from all of them simultaneously. Call me silly, but I just have a feeling that's how it would play out. And call me a hopeless invertebrate, but I realize I am in way no mood to deal. This is exactly why I'm not the president of the United Nations. You're required to take a basic concept like "We don't think you should be stoning women for adultery," and say it just so, all tippy-toe and eggshells, or else everybody gets all bent out of shape, which invariably results in an escalation of hostilities instead of the opposite. On the other hand, if you turn a blind eye so as not to make waves, you're condoning it. Out of deep gratitude that I'm not in charge of world affairs, I try to at least be the kind of citizen who will speak out when I see something that's not right. Had the girls not at this point gotten up to leave, I might have come up with something. Might. As it was, they migrated over to the doors, where they stood throwing candy and gum wrappers onto the floor as their obscenity-rich banter bounced around the inside of the train car as if they each held an invisible bullhorn. I re-enacted the scenario in my mind for days afterwards, trying to come up with different ways I coulda-shoulda-woulda handled it. Even in hindsight, the perfect retort does not present itself. I loathe the idea that someone can calmly go about dirtying, damaging or defacing public property, worry-free that any of the dozens of people directly around them will react. And yet. I said nothing.