Thanks for reporting this, I have encountered this problem too. I believe it is related to the boolean function in the Max SDK that RailClone uses. It's on our list of bugs to resolve, but it may be tricky if it's a Max issue. In the meantime, as you've identified, a quick fix is to reassign the spline. You may also find click Display > Build has the same effect.
I hope that helps, please let me know if there's anything else I can do.