I'd be interested to know where you find this to be true. I have noticed that BART trains through Rockridge don't make much noise. But the track noise is so bad through Fruitvale, you can hear the trains over a mile away.
Looks like it wasn't that clear, but I was referring the elevated lines in Chicago (El) not BART.
I could hear the elevated Bart line at Daly City station two miles away in Colma, where all the graveyards are.