The US standard railroad gauge (distance between the rails) is 4' 8 ½
". That is 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 the same 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
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 made by Roman War Chariots.
Since the chariots were made by or for 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' 8 ½" derives from
the original specification (Military Specification) for an Imperial Roman
Army War Chariot. Mil 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 War Chariots were made to be just wide enough to accommodate the
back-ends of two war horses.