Cecil basically said the article was one part fact to two parts fiction.
I agree. As one that has driven a team of horses, I can refute the idea that it was determined by the horses butt. More acurately, the hoofs and the harness distance between two horses. Too close is unsuitable to various horses, too far apart impractical.
The horse not only set the gauge of railroad tracks, but by the necessity of leather harness they taught us chemistry used in tanning, metal crafts used in biting and harness making. In short, we would not, IMHO, reached the moon by 1969 C.E., had it not been for the horse.