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. 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 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
   State 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 ass 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 war horses.