The one thing that is probably going to keep me trying to pursue this is the fact that, when a train is waiting on the main line, if ANY train leaves the stacker, the train waiting will re-path and pick one of the open spots. I'm not sure what is causing this. I'll will test around and see if I can ...

UPDATE:

I figured it out. I set up a clock to repeatedly count to 200 and put a rail signal just past the entrance.
When the count is < 1, disable the entrance signal.
This flickers the chain signal and when a train is in the block that reads the chain signal, the repath happens.
I was reading ...