How big is the reverse loop? Could it hold two trains? If so, you could have the relay shutdown one section in the reverse loop.
I don't have a full diagram yet, it is a work in progress. I am thinking of using 3 or 4 block sections on the red line. The green would be manually shut off via a switch. I was thinking of using automatic switching when a train is occupied in the green line, but that starts getting more complicated than I wanted for now. I want to start with a basic automatic block configuration so I can have two trains running on the red line and two on the blue line, and they won't crash into each other. Once I get farther along, I'll provide more updates on what I did.
I think my blocks will be 5 to 6 feet long each.