Well, first off, spinning and humming are pretty common stims for autistic people - so maybe she's one of those many, many older adults who didn't get their autism diagnosed. But there are other conditions that might lead someone to behave that way - Tourette's Syndrome, dementia, or Parkinson's disease, for example. She might stay standing up just because it's too hard to get back out of her seat if she sits down. There isn't any way to know, and she might not even know the reason herself - and it doesn't sound like she's any inconvenience or danger to anyone, so it doesn't really matter.
Good for her that she's still getting out and about on the train, even though all those people are pretending not to notice her while talking about her behind her back (she probably knows that they do). That's braver than I usually am when I have a really "stimmy" day - I usually get totally freaked out that people will see me flapping my hands and stay at home!
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When you are fighting an invisible monster, first throw a bucket of paint over it.