First I should mention, that the VRayDisplacementMod will not be rendered on RailClone object if this modifier is applied to used Segment. For this purposes should be 3ds Max's Displace modifier used.
Instead the VRay's VRayDisplacementMod could be applied to whole RailClone object. Just take in mind, that in case of complex geometry this could lead to a high memory usage as the RailClone object will be treated as a one big mesh (instancing will be ignored).
Anyway, there is possible workaround by using Displacement map in VRayMtl slot and adjusting Vray's Default Displacement values.
So, it'll always depend on your scene, which method of applying displacement will be the most efficient.
Hope this helps you a bit.