1. Sharon knew her parents would never approve of her marriage to Ricky because he came from the wrong side of the tracks.
2. Mr. and Mrs. Dawson didn’t want their children to attend Smithson High School because it was on the wrong side of the tracks and it might be dangerous for the children to walk from home to school by themselves.

The expression suggests that towns or cities are divided into a right (i. e. rich) side and a wrong (i. e. poor) side by the railroad tracks that run through them. The expression is often used to describe where someone comes from.