"Night Bus," by Ellen Clair Lamb, in This Time For Sure: Bouchercon Anthology 2021, edited by Hank Phillippi Ryan, Down & Out Books, 2021.
The writing advice for decades has been: Start as far along in the action as you can. If backstory is necessary, you can fill it in after drawing the reader into the story. One result of that is that a lot of the time the mystery is not "Who done it?" but "What was done?"
Jodie is getting on a bus late at night and she wants to be left alone. Unfortunately the last seat open is next to a chipper old lady who is eager to chat. Her name is Barbara and she is observant, too observant for Jodie's liking because Jodie has a secret to keep. And that secret - what was done? - will keep you turning pages.
Just like last week (and from the same book) it is the last sentence that made this story my favorite. Very clever tale.