100Hz frequency cab signal systems were instituted by the Pennsylvania Railroad (PRR) in the late 1920’s and are only associated with the 4 signal aspect of the railroad industry. This change in signal detection resulted in a 1.25 to 3 Hz on-off pulsing of the signal, which would be used as a code to convey the aspects. This new 100Hz system allowed four signal aspects: Restricting; Approach; Approach (next signal at) Medium (speed); and Clear. Initially the cab signaling system only acted as a form of automatic train stop where the engineer would have to acknowledge any drop in the cab signal. Later, passenger engines were upgraded with speed control, which enforced the speed rules associated with each cab signal (Clear = No Restriction, Approach Medium = 45 mph, Approach = 30 mph, Restricting = 20 mph).
How Does 100Hz work for Rail Systems?
This fail-safe system of pulse code cab signals at 100Hz work by sending metered pulses along an existing AC track circuit operating at some chosen carrier frequency. The pulses are inductively detected by a sensor mounted above the rail before the leading set of wheels. The codes are measured in pulses per minute and for the 4-aspect PRR system are set at rates chosen to avoid any one rate being a multiple of another, which may cause reflected harmonics causing false signals.