If you are a “Main Liner”, in or around Philadelphia, then you really belong.
You are a member of the Upper Crust, one who can look down your nose at any other class of persons in Pennsylvania, at least, and perhaps elsewhere, except, possibly, a person from Boston’s Back Bay or an F.F.V. (First Family of Virginia).
That is to say, by way of explanation, you live along the “main line” of the Pennsylvania Railroad, in the beautiful suburban area just outside Philadelphia.
The cognomen for these highly aristocratic, ultra-conservative members of society has been in use since early in this century, but the distinction is fading, as elsewhere.