When I see things like that I kind of assume there’s some addiction or mental health issues going on. It’s not typical behaviour for someone who’s well-socialized and obviously difficult to be around but I’d suspect that persons got much deeper issues and public shaming isn’t helping them.
Trust me, there are people in this world who care for noone but themselves. I think you've just been lucky enough to avoid meeting one of these people.
This is because of capitalism and fast pace of life. Some use transportation time to do a lot of things they can't do otherwise like sleeping, cutting nails or applying make up. Some also to work or study.
Yeah, assuming her feet don't smell I'd have no issue with her cutting her nails on the train-- if she was catching them with a plastic tray, baggie or even a napkin so they don't fall on the seat someone else is about to sit down on. But she's not doing that, which is gross and disrespectful to your fellow passengers, IMO.
This is no different than shaving my body hair on the train. I’m forced to leave my trimmings on the train too because my boss wants me in the office 8:00 on the dot!