I have modified my drawing to show how relay can be used to stop a following train. When relay is open and signal is green, power flows to the center rail of track approaching green signal. When Relay is closed, power is cut to the center rail, stopping train, until previous train clears the insulated rail block. Again, I am showing only one track length,the insulated rails, outside rail to control relay, and insulated center rail to stop following train can be longer.
I can't speak to how this would effect command control or other modern control systems. I am a Post War, conventional operator.
Steve