Bit weird but ive just been looking at that today, From what Ive found (not finnished yet tho) Basicly for the open bearings used on supported rail then depending on the manufacturer the rated load is only if applied vertically from above, side loads reduce it to 70% and upward forces at 50%

So if the pillow block is mounted on its side then its vertical capacity is reduced to 70-75% but the bigger problem is the open face which is reduced to 30-50% capacity. As the open face is now in a direction of travel you effectivly have full rated strength in one direction but only half in the other.

If the rails are mounted as a pair, back to back ( ie top and bottom) then the weak spots are evened out, and at first glance you end up with an approx 75% reduction from the rated load capacity in all directions.

As I said not finished yet but will post some pic's latter to see if I'm on the right track.