Gongs are present on rail vehicles , such as trams , streetcars , cable cars or light rail trains, in the form of a bowl-shaped signal bell typically mounted on the front of the leading car. It was designed to be sounded to act as a warning in areas where whistles and horns are prohibited, and the " clang of the trolley" refers to this sound. Traditionally, the gong was operated by a foot pedal, but is nowadays controlled by a button mounted on the driving panel. Early trams had a smaller gong with a bell pull mounted by the rear door of these railcars. This was operated by the conductor to notify the motorman that it is safe to proceed.