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I can answer about Amtrak's cab code system, and, this may apply to most other railroads. Coded AC voltage pulses are placed onto the rails. Located under the engine, or, the controlling cab, of the train are pick up coils that are placed ahead of the front wheels and about 8 inches or so above the rails. The electrical pulses on the rails are coupled to the train's pickup coils in the same manner a transformer works. The pulses are decoded by the on-board cab code system to display the proper cab code signal inside of the operating cab of the train. For a clear signal, we place 100Hz of AC energy onto a set of contacts in a relay. The coil of the relay will be controlled by pulses of 12 or so volts DC. The contacts inside of the relay are then moving at the same rate as the DC coded pulses that are placed on the relay's coil. The internal contacts of the relay are then making/breaking the 100Hz AC energy at the coil's applied code rate and then the "pulsed" 100Hz AC is coupled to the rails via a transformer. For a clear straight move, we would send out 180 code, or, 180 beats per minute, of coded AC onto the rails. For a crossover move (approach medium), 120 code would be transmitted onto the rails. For an approach signal, 75 code would be transmitted. A Restricting cab signal occurs when there isn't any code on the rails (or, an out of spec. code is detected). This is just a basic explanation, and, since I am new on here, I don't know how many words a reply post can have. Please feel free to e-mail me for any other questions. I am still searching for cab code signal info on the UPRR and Sante Fe.
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