Hi, RailClone 2.7.4 is not compatible with V-Ray 3.6.
You get those errors because the instancing engine cannot initialize V-Ray. As consequence, instances are not generated and the RC object is generated as a huge mesh.
So, the error message is not only a warning.
But we just released RailClone 2.7.5
with support of V-Ray 3.6, specially for customers who doesn't want to upgrade to RailClone 3 yet.
I hope that helps.