 In some downtown tunnels, cartoons depicting an advertiser's product were mounted on the walls of the tunnel at the level of the cars' windows.A retail film processing outfit called Direct Film advertised on the north wall in the Westbound track of the Guy (now Guy–Concordia)-to-Atwater Station (Green Line) during 1967–1969.Strobe lights, aimed at the frames of the cartoon and triggered by the passing train, sequentially illuminated the images so that they appeared to the viewer (passenger) on the train as a movie.