Take a close look at the tunnel mouthes made by rail-sim and andi06. RS have used the tunnel spline where Andi has used the Spline Eraser.
Here's my best crack at an explanation. There is a tunnel that can be attached to the tunnel mouth, therefore there needs to be a "fixed track" for splines attached to the tunnel. The tunnel mouth itself does not need the additional tunnel, so the spline attachment remains invisible. I'm not sure that I'd understand my own explanation, but hopefully it'll show you where you need to look to find the answer - I'm fairly sure you'll see it once you know where to look.
Perhaps if Mike's about you could ask him, since he should really know.