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While old movies often show parts of Chicago that have changed or disappeared, there are some aspects of the city that remain constant. The protected lakefront is frequently featured, and directors love to have their characters drive along the shadowy streets underneath the elevated train tracks.

But, for the true Chicagoan, a surprising reminder of how unchanging the city can be comes near the end of the 1953 film “City That Never Sleeps.” A cop chases a murderer onto the elevated tracks, and, as they’re wrestling around within inches of the electrified third rail, a small rectangular sign is clearly visible at track level: “DANGER: ALL PERSONS PROHIBITED FROM ENTERING UPON OR CROSSING THE TRACKS.”

For more than 50 years, generations of Chicagoans have seen that same sign at Chicago Transit Authority stations throughout the city. In recent years, thanks to the CTA’s station and track rehabilitation program, many of those signs have disappeared.

But at least one remains today at the Sheridan Road station on the Red Line.