The boolean operation in RailClone is carried as a last step after the array has been built so I'm afraid there isn't away to detect which segments have been sliced and use that information in the graph.
If you are using V-Ray there may be a workaround that used the VRayDistanceTex to perform a render-time boolean and capping effect. To use it you can to follow these steps.
1 - Create your array as usual but turn off Slice
for the Tile segments
2 - In the generator's settings change the Clipping Area > For No Slice
setting to Preserve
3 - Create solid geometry to represent the area you want to remove from the tiles. Any part of the tile inside this volume will not be rendered. Open the geometry's properties and turn off Renderable
4 - In the Tile Material, add a VRayDistanceTex and turn on Inside Separate
and Inside Solid
. Set the distance to 0
5 - Add the boolean volume created in step 3 to the VRayDistanceTex's Objects list. If you render now you should have a render time boolean. This has the added bonus of retaining full instancing.
6 - To add the caps, duplicate the boolean geometry and apply a new cap material.
7 - Create the cap material however you like, but add another VRayDistanceTex map to the opacity slot. As before turn on Inside Separate
and Inside Solid
. Set the distance to 0 and this time set the Inside Colour
8 - Add the RailClone objects to the VRayDistanceTex's Objects
If you render now you should have booleans with capped ends that have a separate material.
For an example of this set-up please see the attached file.
I hope that helps but please let me know if you have any further questions.