There was a post on the old forum to which martinvk replied. He suggested looking at existing spline assets to see how they are constructed. The OP, Rockster, replied that he had been successful.
Although I have not tried to make a spline, examination of the several fences and walls etc indicate that the config file needs the contain some particular tags:
bendy 1,
kind is track or bridge
length
endlength
width (thickness of spline)
height
Have a look at "Corrugated Iron Fence" by Vulcan.
Hope this helps
Trevor