Q: Where did the term “red-light district” originate?
A: Many people believe the phrase “red-light district” goes back to the railroading days around the beginning of the 20th century. Railroad crews used red lanterns for signaling trains, so they would use these lanterns to light their way as they left the train yards.
When they visited a prostitute, the men would leave the lanterns on the porch of the brothel or prostitute’s house while they were inside, often to signal she was busy.
Another explanation dates back to the 1890s, when prostitutes would sometimes put red shades on candles or electric lamps and place them in their windows to discreetly advertise their trade. The practice continues today in some places, most notably in Amsterdam.
Source: Travel guru Rick Steves’ Web site, www.ricksteves.com