This little piece of trivia is something I read many years ago.  I copied it at the time, but I failed to note the original source.  Appropriately, the subtitle was “Really Being in a Rut.”

The U.S. standard railroad gauge (the distance between the rails) is 4 feet 8.5 inches.  That’s an exceedingly odd number.  Why was that gauge used?  Because that was the way they built them in England.  The first U.S. railroads were built by English expatriates.

Why did English expatriates build them like that?  Because the rail lines were built by the same people who built the pre-railroad tramways, and that’s the gauge they used.

Why did they use that gauge then?  Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing.

Okay!  Why did the wagons use that odd wheel spacing?  Well, if they tried to use any other spacing the wagons would break on some of the old, long-distance roads, because that’s the spacing of the old wheel ruts.

So who built these old rutted roads?  The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions.  The roads have been used ever since.   And the ruts?  The initial ruts, which everyone else also had to match for fear of destroying their wagons, were first made by Roman war chariots.  Since the chariots were made for or by Imperial Rome they were all alike in the matter of wheel spacing.

Thus we have the answer to the original question.  The United States standard railroad gauge of 4 feet 8.5 inches is derived from the original specifications for an Imperial Roman army war chariot.  Military specifications and bureaucracies live forever!

So, the next time you are handed a specification and wonder what horse’s ass came up with it, you may be exactly right.  The Imperial Roman war chariots were made to be just wide enough to accommodate the backends of two war horses.