You probably don't encounter the same people all the time. There's enough people out in the world such that each one can "have their moments" when they're around you, and nowhere else, and you wouldn't know the difference.
I do sympathize with you though. I've got a sound sensitivity problem, and lots of noises bug me way way way out of the ordinary compared to most peoples' tolerance. I know this isn't specifically related to quiet cars on trains, but noises in general still bug me (even my own oftentimes). I'm sometimes shocked how I can be on an airplane, with gigantic engines blowing away at tremendous force just 20 feet from me, yet a person a row ahead of me chewing gum or chowing down on pretzels penetrates in to my brain.