Thanks for scene file.
First, as You are using Corona, please disable Instancing engine. This is needed for some features to work with Corona renderer.
Please refer to ForestPack
documentation to learn more which features are supported/partially supported.
You very rarely need to add additional modifier on RailClone object.
Remove Your UVW mapping Add modifier:
In this case it is best to add UVW mapping on Your segment ant tell RailClone to use it.
I have also disabled capping segments, as that messes UVs of capped segments:
And for more variations, lets add mirror and stepped rotations macros:
Attached file for Your deeper investigation. If You have any questions, let us know.
We have a video tutorial
covering all the information needed. I hope You can find it relevant.