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!