In that case, I wouldn't use a direction marker at all. You have to be careful using direction markers. Overuse of them will result in a lot of "no path to destination" messages when using the AI.
Route finding for the AI drivers has probably been one of the most talked about subjects here. Basically, it's sort of like putting a monkey at the controls. He may get there eventually ("may" being the operative word), but you will be left scratching your head on how he got there.
Actually, considering this is just an entertainment simulation, the AI drivers aren't too bad, if you keep it simple. Some people go completely bonkers when the AI won't run a route exactly. I accept it for what it's capable off and live with it. I use the AI mostly for trackmark to trackmark running. For instance running a through train from staging to a yard track. I may have the engine uncouple and go to a service track, but I will do most of the switching myself.
With a little practice, you will learn what you can get away with, and learn basic signaling. It has a lot to do with AI path finding.
John