I discussed this issue with the Corona team, but i'm afraid it would be very complex to support motion blur in RailClone, while maintaining instancing.
That requires some limitations, as the output instances and its topology doesn't change along the animation.
But we cannot assume that, because a single change in RailClone may generate a completely different result, both in the number of segments and their geometry.
Your case is very special, because really you are using RailClone as a scatter, but there is no simple way to detect this situation from others much more generic.
On the other hand, motion blur can achieved applying a mesh modifier to the RailClone object. Obviously that breaks instancing, but it may be useful in some cases.