The standard railroad gauge (distance between the rails) in the United States is 4 feet, 8.5 inches, a somewhat 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 build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that was the gauge they used.

Why was that gauge used for trams? 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, but 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 the Roman legions, and the roads have been used ever since.  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 all made either 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.  The next time you are handed a specification and wonder what horse's ass came up with it, you may be exactly right: Imperial Roman chariots were designed to be just wide enough to accommodate the back-ends of two war horses.