If you just need to reduce the poly count so that it can be more easily processed through the stack then there might be a couple of workarounds. First of all it depends on if your camera is animated or not. If it's not animated you could draw a simple clipping spline to match the camera frustum and use this to show only those segments that are in view. The process for that would be as follows.
1 - Draw a spline from the top view the size of the camera frustum, ensuring that it includes all the parts of the RailClone object that are in the final render.
2 - Add a clipping spline to the generator(s).
3 - Change the clipping mode to Include.
4 - For more efficiency, and if the style allows it, you should also turn off slice for all the segments. Set the generator's No Slice
mode to Preserve
to ensure you don't lose geometry around the edges of the shot.
Unfortunately if your camera is animated this won't work because animated splines do not currently update clipping operations. Instead you could just try a simple volume select to delete those parts of the mesh that aren't visible before you pass them up the stack. To do this.
1 - Create a geometry object that matches the camera frustum. I find the ViewFrustum
script by Martin Breidt really handy for this, it generates a geometry object that is linked to the camera, so if you change the field of view then the geometry object also updates.
2 - Turn off the RC object's instancing engine.
3 - Add a Volume Select
4 - Change Stack Selection Level
5 - Change Selection Method
6 - Add a DeleteMesh
I've attached a sample file illustrating both techniques.
I hope that helps, please let me know if you have any further questions.