Interesting, thanks again. This also settles the question of whether or not opposing signals are "supposed to stay red" behind trains in CTC.

Would these magnetic stick relays have been something like GRS KP relays? And would there be one at each signal, controlled and intermediate, or just at the "head blocks"?

What I was looking at was a means to apply codes directionally on single track. They would be transmitted only when a TS is occupied -- for cab signal usage (i.e. line wires would be required for signal control). Since the direction of traffic shifts back and forth, there would have to be means of interchanging the TT end with the TR end so the codes would always be sent to the front of the train. Reverse codes, polar codes, etc.; it can get complex (and I don't have any wayside decoding apparatus available).