For a car to enter the loop, one has to leave it to make a space so if a car needs to the loop for unloading, so I don't think they need to know in advance that a car needs the loop to unload. When the car to be unloaded reaches the end of the main loop, they would move it on to the outside loop, and then as it's gap comes around a car waiting on the outside loop would be dispatched into the gap. So, conceivably, the CM working the unload could signal when a car needs to be pulled to the outside loop.