the other side of the tracks
A part of a town or city that is particularly impoverished (and usually dangerous or undesirable as a result). "Tracks" refers to railroad tracks, which are sometimes thought of as demarcating different economic areas of a town. I was always looked down on as a kid because I grew up on the other side of the tracks. His mother didn't want him dating anyone from the other side of the tracks.
other side of the tracks
and wrong side of the tracksthe poor part of a town or city. He was a rich boy, and she was a girl from the other side of the tracks. You don't want to buy a house in that neighborhood. It's on the wrong side of the tracks.