1
general::
The desirable part of town, as in They were relieved to learn that his fiancée came from the right side of the tracks. This expression alludes to the fact that when a railroad ran through a town, it often divided the prosperous neighborhoods from the poor ones. The latter district was called the wrong side of the tracks, as in The children from the wrong side of the tracks often came to school without having eaten breakfast. Today these terms are considered snobbish. [Second half of 1800s]
American Heritage Idioms