The US Standard railroad gauge (distance between the
   rails) is 4 feet, 8.5 inches. That's an exceedingly odd number. Why
   was that gauge used? Because that's the way they built them in
   England, and the US railroads were built by English expatriates. Why
   did the English people build them like that? Because the first 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. Why did the wagons use that odd wheel spacing? Well, if
   they tried to use any other spacing the wagons would break on 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 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 questions. The
   United States standard railroad gauge of 4 feet, 8.5 inches derives
   from the original specification for an Imperial Roman army war
   chariot. Specs and Bureaucracies live forever. So, the next time you
   are handed a specification and wonder what horse's behind came up with
   it, you may be exactly right. Because the Imperial Roman chariots were
   made to be just wide enough to accommodate the back-ends of two
   warhorses.