The American-English phrase the wrong side of the tracks and its variants denote the socially inferior part of a town.

This phrase alludes to the division of a town into areas separated by railway tracks. The U.S. author Thorne Smith (1892-1934) described this social phenomenon in The Stray Lamb (New York: Cosmopolitan Book Corporation, 1929):

—as reprinted by Pocket Books, Inc., New York, in 1948:
As if preordained by a class-conscious God with an eye to real estate values, this fair mansion was situated on the financially correct side of the tracks.
In most commuting towns of any recognized worth there are always two sides of which the tracks serve as the line of demarcation. There is the right side and the wrong side. Translated into terms of modern American idealism, this means, the rich side and the side that hopes to be rich.
On either side of the tracks there sometimes extends a quarter—a blot—that is not rich, will never be rich, and makes no visible effort to be rich. The blot thrives squalidly amid its fights, sufferings, and enjoyments. It is fundamentally superior to either side of the tracks, because it envies neither, regarding all members of the community as legitimate prey.
Properly speaking, however, those who dwell on one side of the tracks form a separate and distinct race from those who have their being on the other side. The rich side is naturally of finer clay, superior morally, physically and intellectually. And it is the bounden duty of those who dwell on the rich side to defend its borders against the untimely incursions of the financially striving side. Between the two a silently genteel yet none the less bitter guerrilla warfare is in constant progress. No pickets are visible, no orders to halt are audibly voiced, no hostilities are openly exchanged. Nevertheless, there is a certain sense of vigilance. They shall not pass, is the order of the day.

These are the earliest occurrences of the phrase the wrong side of the tracks and variants—unambiguously used in the above-defined meaning—that I have found, in chronological order:

1-: From The Whirl of Society, published in The Inter Ocean (Chicago, Illinois) of Wednesday 24^th June 1903:

There was one wedding of consequence yesterday. A mighty pretty one, too, and deftly arranged and admirably executed. The bride was Miss Bessie Swift, daughter of Mr. and Mrs. Louis Franklin Swift, and the groom Mr. Charles A. Fernald of Santa Barbara. The ceremony took place at 8 o’clock at the home of the bride’s parents in Lake Forest. Miss Swift is an extremely popular young woman. She has a host of friends in Chicago and Lake Forest. In the latter place every one is friendly, unless you happen to reside on the wrong side of the railroad track.

2-: From the sports news, by T. H. Murnane, published in The Cincinnati Enquirer (Cincinnati, Ohio) of Sunday 1^st January 1905:

Dick Cooley did a turn at a variety theater here last week. As he came from the wrong side of the railroad track, the applause for the league man was very light.

3-: From A Rebuke to Intolerence [sic] and Phariseeism, published in the Berkeley Daily Gazette (Berkeley, California) of Wednesday 19^th April 1911:

The Berkeleyans living outside the immediate precincts of the University part of the town, the dwellers in the unfashionable lowlands and those settled on the wrong side of the railroad track, found early that they were not regarded as Berkeleyens [sic] except for political purposes around campaign time.

4-: From The Nebraska State Journal (Lincoln, Nebraska) of Saturday 27^th January 1912:

The effort to vote bonds in the sum of $100,000 for the building of a court house big enough for the business of the county was the last project to fail because of the ancient and honorable feud between the contending sections. It was provided in the call for bonds that the new court house should be erected on the site of the old, which is a long way from the present business center and, as a considerable majority believe, on the wrong side of the railroad tracks to exemplify the greatest good to the greatest number.

5-: From Stories by Edna Ferber ¹ Reflect Life: She Writes of Plain Everyday People, by Annie Laurie, published in the San Francisco Examiner (San Francisco, California) of Monday 13^th September 1920:

Emma McChesney […] is just one of a dozen new characters that Edna Ferber has picked up right out of life and put into stories for us.
Don’t you remember “Buzz Somebody” the touch [misprint for ‘tough’?] guy who was the admiration of all the young village loafers? The one with the pimples and the “sweet running engine,” and the affair with the loose mouthed village siren, who lived on the wrong side of the track.

¹ Edna Ferber (1885-1968) was a U.S. novelist, short-story writer and playwright.

6-: From the account of a speech on the public schools that the U.S. author and educator Dallas Lore Sharp (1870-1929) gave to the Boston Rotary Club—account published in The Boston Daily Globe (Boston, Massachusetts) of Thursday 24^th November 1921:

“I know,” he went on, “that many parents would rather see their children dead than have them go to school with the other children of the same town. Death is preferable to associating with people from the other side of the railroad tracks.”
[…]
“I went to live in Hingham 20 years ago, on the wrong side of the track. If you live on the wrong side of the track in Hingham, you can’t even get fire insurance.”

7-: From an article by C. W. Gilbert about the U.S. Senator William Squire Kenyon (1869-1933), who had just “quit the upper house in disgust”—article published in the Greensboro Daily News (Greensboro, North Carolina) of Friday 3^rd February 1922:

The senate is something more than a body which meets daily in the capitol or frequents committee rooms. In its off hours it is a social body. More than half of the tie that binds its members together are cemented at dinner parties. You either belong to the senate set or you do not. You are likely not to do so if you cannot be counted upon to vote right.
It is like a small suburban town. If you live on one side of the railroad track you belong to the country club and take part in the town’s social life. If you live on the other side you do not. The line is sharp. It is an unpleasant line.
Mr. Kenyon lived on the wrong side of the track politically. He did not belong.

8-: From the account of a speech that George B. Utley, President of the American Library Association, made during this association’s convention, in Chicago, on Saturday 27^th January 1923—account published in The El Paso Times (El Paso, Texas) of Sunday 28^th January 1923:

“Each little community is a metropolis on a small scale. Each has its own little Bohemia with candle-lighted tea rooms, bobbed hair and parlor bolshevism; its own literati; its 400, and its masses.
“In each community it makes a difference if you live on the wrong side of the railroad tracks. But the middle west will be found more responsive to good literature than the once cultured east.”

9-: From the Dayton Daily News (Dayton, Ohio) of Sunday 18^th March 1923:

Whenever one thinks of short stories, one thinks of Waller B. Pitkin ². Professor Pitkin is publishing through Harcourt, Brace and Company this week a collection of representative stories by some of America’s youngest writers which he entitles “As We Are.” All of these stories are from American life and show the deepening of class lines which is a striking tendency of recent years. They show this in many aspects—the predicament of the Alabama professor who finds himself in the same Chautauqua circuit with the Negro Jubilee singers; the girl whose only sin has been that her parents live on the wrong side of the railroad tracks ³; the old aristocracy trying to keep aloof from the man “on the make;” and the hundred and one little distinctions and handicaps that are making us “as we are.” The book is interesting also as showing what the youngest of our fiction writers are doing; many of the names in this volume are going to figure prominently in American writing.

² Walter Boughton Pitkin (1878-1953) was a U.S. author and university professor.

³ This is an extract from Railroad Tracks, by Emanie N. Sachs, as published in As We Are. Stories of Here and Now. Collected by Walter B. Pitkin (New York: Harcourt, Brace and Company, 1923):

The man, sitting behind the clear glassed lobby at the Merville House, had watched Linda walk by, and had obtrusively followed. He wouldn’t go away, and persisted in his efforts to speak to her. Then Dick, who was on his way to the station to inspect the Marvins’ new car, intervened. A word had been enough. And Dick walked home with Linda to protect her from further annoyance. He had made an engagement for the next night, and eventually for the Fair Hop. His mother’s guest for whom he had been booked happened to be ill at the last moment, so he was free. Linda had hesitated before Dick fairly made her accept. She knew it was an uptown affair, but what could it really matter whether your house was on one or the other side of the railroad tracks?

10-: From Breezy Bits, by ‘Ben’, published in the Elmira Star-Gazette (Elmira, New York) of Friday 1^st August 1924:

Rattlebrain, Nev.—Oscar H. Sapp would neither deny nor affirm charge he has cast his lot with Peggy Joyce. It ain’t much of a lot anyway, being located on the wrong side of the railroad tracks, and Peggy probably wouldn’t be interested.

11-: From a text by one William M. Maupin, published in The Omaha Morning Bee (Omaha, Nebraska) of Wednesday 19^th November 1924:

Chicago’s gang war may be provocative of more fatalities, but it is not in it with the gang war that raged in a certain Missouri town during our boyhood days. Our gang war did not result in sudden death, but the amount of gore spilled was something frightful, at least to our mothers. Nobody knew how the trouble started, for the boys inherited it. All we knew was that the railroad was a dividing line, and it was a foolish lad from the east side who ventured alone to the west side of the tracks, and vice versa; although we didn’t know what vice versa meant then.
Unfortunately for us east siders, the ol’ swimming hole and the woods where the hickory nuts and walnuts grew in greatest profusion was on the west side. We certainly earned our swims and nuts. By common consent the ball grounds on the east side were neutral ground, but there was nothing neutral about the ball games. A nine-inning game between the two factions meant an average of 18 umpires, unless we could persuade some grown man to officiate in that capacity.
In these football days we hear a lot about the speedy boys who carry the ball. They run like they were hitched to a post compared with one of us young fellows caught alone on the wrong side of the tracks.