Paul,
Here you go, I've attached the railclone model, as I moved up a level I encountered the same problem as previous
I selected the original spline, copied it on the Z axis, added a linear generator for this spline set to scale and added my segment.
I can find a way to work around this, but it's frustrating it's occurring and I'd like to understand what is causing it, as it seems to be happening more lately.