Steam locomotives used whistles for well over a century to warn people near tracks, signal departure and communicate with railway staff, and that long association is why the sound still reads instantly as a train even without a picture. As steam engines were replaced by diesel and electric trains, the whistle became a symbol of an earlier age of rail travel, preserved in films, documentaries and sound libraries. Online it survives as a short, recognisable effect for creators who want an old-fashioned railway atmosphere without recording a real engine.