Sure, you could just sit there and say nothing, but you need to make eye contact with someone who doesn't sprinkle his dinner with human feces to establish that you are sane, and your loud new friend is not. Of course, you never want to antagonize a crazy person, so you just get out the "Well ..." and then you roll your eyes or do a subtle head nod to convey "LOOK AT CRAZY!" Yep, good job. You're totally sane now. You and your buddy on the train are now official partners in not being crazy. And best of all, the crazy dude can't accuse you of saying anything. Yep, crazy people lose again.
The "Would You Like ... (My Seat)?"
Here's another that works well on subways, but really it's a keeper in any situation with more people than seats. These days I don't offer my seat to young women. Why? Because I'm a feminist, that's why! But I do offer my seat to moms with little kids who need to sit on their lap, or the handicapped, or pregnant women, or really old people. I think that's as it should be, but let's not argue. That's not the point. The point is how you do it. Sure, I could do it with a full sentence that's easily comprehensible. But I don't. I don't know why, but I never say, "Excuse me, but would you like my seat?" or "Would you care to sit here?"
Instead I do this fun thing where I start the sentence and then just point. Typically it's to the seat, but sometimes I'll assume a very cool half-crouched position with raised eyebrows while seemingly pointing backward to that seat. Now, just because it might look like I'm suggesting that an old lady engage in oral sex with me is no reason to doubt the validity of this technique.
"Are you asking me to stand or kneel?"
Do you ever want to say no to someone, but you're weak-willed or manipulative or just trying to be polite? If so, then the "so" rejection is a great sentence fragment for you! Let's say someone invites you to do something you don't want to do. You could say "That's a terrible idea" or "Get away from me, freak" or just "No." But then you'd have to face the consequences of your feelings.