Hi,
Thanks for the question. This is actually rather tricky to achieve in RailClone because it is designed to create objects using rows and columns and this requires something more like a random scatter or a packing algorithm. Especially because it seems a cylinder should be placed where each ring intersects, which RailClone cannot detect.
Nonetheless I attach a file which achieves something similar by randomising the rings and their sizes and then uses negative padding values to get them to overlap. To create a "scattered" effect I've randomised the vertical transform value.
To create the area I have rotated the RC object 90 degrees so that it's Z axis points towards the
surface. You can then extract the surface's boundary spline and use it as a
clipping area. As RailClone may give artefacts when the clipping spline is the same size as the surface, I've extended the surface's boundary slightly.

Please take a look at the attached file, and please let me know if there's anything else I can do to help out.
Many thanks,
Paul
ps. If you want to use Forest then you may find this
trick using VRayDistanceTex works and has the benefit of maintaining instancing.