Hi, when "Extend to X/Y Size to Area" is enabled, RailClone builds the array using the bounding box of the Clipping spline.
By default the array is aligned at the origin point of the bounding box, and there is not a specific option to "reverse" it, but it can be done in two ways:
Option A: With Extend to X/Y Size to Area enabled:
1) Set Clipping Area->Z Rotation = 180 to reverse the array.
2) Set Clipping->Expand to modify the offset. This is not very precise because modifies both X and Y, but may be useful in some cases.
Option B: With Extend to X/Y Size to Area disabled:
This approach is more precise, and lets you to adjust the array manually.
1) Disconnect temporary the Clipping Area node from the Array (to see the full array while we adjust it).
2) Create a straight X spline larger than the base of the ceiling. Assign it to X Spline.
3) Set Geometry->Y Size, to a value larger than the ceiling's Y size.
4) Move the X Spline and adjust Y Size to cover the ceiling's area completely.
5) Reconnect the Clipping Area node.
To reverse the array, just select the X Spline and reverse it from Max.
Please note in this case you must move the X Spline to the top edge, because the array is rotated. Or optionally, set a negative value to Y Size.
I hope that helps.