100Hz Frequency Converters:
A 100Hz frequency converter is needed because the railroad utilized 100 Hz for safety. The 100Hz 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. However as the US operates at 60Hz power, a converter is needed to change 60Hz to 100 Hz. Therefore, a 100Hz frequency converter has to be utilized.
This change in signal detection resulted in a 1.25 to 3 Hz on-off pulsing of the signal. This 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. Subsequently, the engineer would have to acknowledge any drop in the cab signal and take action. However, passenger engines were upgraded with speed control. As a result, it 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:
The fail-safe system of pulse code cab signals at 100Hz works by sending metered pulses along an existing AC track circuit operating at some chosen carrier frequency. The result is the pulses are inductively detected by a sensor mounted above the rail before the leading set of wheels. As a result, the codes are measured in pulses per minute. For the 4-aspect PRR system, they are set at rates chosen to avoid any one rate being a multiple of another. The reason for this action is to avoid a reflected harmonics causing false signals.