Thanks again for the suggestion. I'll add it to the wishlist but it would involve some big changes to the way in which RailClone works. In the meantime you could remove items using clipping splines but I'm guessing you need more control than that. I have another alternative that may work for you though, how about converting RailClone to a Forest Pack object so you can control the distribution using maps? This would involve a few steps:
1 - Convert RC to instances using RailClone Tools.
2 - Use the InstancesToForest
script to convert the RailClone generated insances to a FP object.
3 - Add a surface
in the Surfaces rollout
4 - Apply a map
in the Material Rollout > Get Colour from Map slot
. Change the mode to As Texture
5 - Import the DenstityByTint
Forest effect (attached to this post). This will allow you to control the density using the greyscale values contained in the map used in step 4.
Here's a little video demo
I should point out that this will only work where full instancing is retained in RailClone. If segments are deformed or sliced they are no longer instanced and will not be added to Forest Pack. I hope that helps out. Please let me know if you have any questions about this approach.