I do occasionally ask people chatting loudly in the quiet carriage to speak more quietly, and to my everlasting surprise it sometimes works.
Last long-distance, ruinously expensive train journey I made, I was in the quiet carriage. Also in the quiet carriage were:
- a party of four adults not even sitting together (there was plenty of room, they were just spread out), having loud conversations across the aisle;
- a couple with a toddler who, when she wasn't shrieking, was running up and down the aisle with one of her parents;
- assorted twenty-somethings travelling alone, either talking on their phones or with tinny music leaking out of their earphones.
I snapped. I was incredibly polite, but I was firm. Amazingly, I got apologies and they all mostly shut up.
(I think when it's that bad it's easier to say something because you are at your wits' end and don't care so much about being awkward or embarrassing.)