Way back, I believe it was more common to use a leather cord to connect the signal bell from the engine to the cars, as well as through and between the cars. There was some sort of clip arrangement to connect between the engine and the first car and so on through the train. The bell arrangement was succeeded by an air whistle in the cab actuated by a valve in any car in the train. (I remember the distinctive "peep, peep" of the cab whistle.) In some cases, an electric buzzer was used. Amtrak has a buzzer system in its trains, I believe.