Hi, This is because you have an empty segment node that has no X Size. If you tried to distribute a zero-sized segment as a Y Evenly segment it would cause an infinite loop that freezes the program. To prevent this, RailClone will simply stop the array building when it detects this issue instead. Overriding a segment with an X Size as you have done is a way of working around that.
I hope the makes sense,
Paul