Ben Hammersley.com: The connection between the space shuttle and a horse's arse.

"The U.S. standard railroad gauge of 4 feet-8.5 inches derives from the original specification for an Imperial Roman war chariot.
So the next time you are handed a specification and wonder what horse's ass
came up with it, you may be exactly right, because the Imperial Roman war
chariots were made just wide enough to accommodate the back end of two war
horses."