That was such an insane thing to discover my first time on the BART. I’m sitting there thinking a catastrophic accident is about to occur, but everyone else is just on their phones... oh, that’s just the screech!
Although I also have sensitive ears. :/
I went as far as lodging an official complaint and was told that to deal with capacity trains are running more often and faster than they have in the past, hence the increased noise. Furthermore due to the Northern Line having been built before environmental noise regulations were passed, it's exempted so Transport for London aren't planning on doing much about it.
At the risk of sounding like a curmudgeon - between always on ear buds and mass transit noise we're risking a generation of kids suffering hearing loss well before their time.