The problem is that when a train happens to wait to long at the signal after it, then it turns around, as you probably know. When that happens and there's a one-way signal it stops at that point to follow the correct route again. With a both way signal it CAN run through the last section it was instead of going back the follow the correct route again and can mess up the whole system. A trick I often use is using the gantries as eye candy and putting a small dwarf signal in front of it to make it a one-way signal, the gantry is actually not used as a signal in that way, but in my opinion it looks better like that.
Don't have access to my screenshot folder at the moment, but when I have and you're still interested I could post one.
@IA
Oh god, should have known that years ago. I have been hassling with that since I've got it around the release date.
