Railroad Signal System

a system of sight and sound signals for transmitting orders and instructions to ensure the safety of train traffic and of switching operations. Sight signals include permanent signals, such as light signals and semaphores, switching panels, and warning disks; and portable signals, including panels, flags, lanterns on poles, and rockets. Hand signals or train signals can include flags, disks, and lanterns. Additional sight signals are given by signs and signposts. The main signal colors are red, yellow, and green. In signaling connected with train movement, a red light means motion not permitted, a yellow light means reduced speed, and a green light means motion permitted. Dark blue, pale white, bright white, and milky white (normally burning, normally nonburning, flashing, and nonflashing) lights are also used. Sight signals are classified as around-the-clock, day, and night lights.

Sound signals are given by locomotive and hand whistles, by station bells, and by rockets.