The U.S. Standard railroad gauge, that is, the distance between the rails, is 4'8½". This is an exceedingly odd number.
Why this number? Because that is the way railroads were built in England, the birthplace of railroads. It was English locomotives which powered America's first railroads.
Why did the English build their railways to this gauge? Because the first rail lines were built by the same people who built the tracked tramways which preceded the railroads, and they used this gauge.
Why were they built to this standard in England? Because the people who built tramways used the same jigs and tooling used to make wagons, which used this wheel spacing.
Why were wagons built to this wheel spacing? If any other wheel spacing were used, the wagon would break on some of the old long-distance roads in England because that was the spacing of the wheel ruts.
Who built these old rutted roads? The Romans built the first long-distance roads in Europe and England for their Legions. These roads were built of stone, much like a low, wide wall set in a trench. These roads are still in use to this day.
Where did the ruts come from? The Roman Legions used wagons and war chariots and it was the passage of these that originally rutted the roads by wearing grooves in the stone paving. Other wheeled vehicles had to match this spacing to keep from breaking their wheels. The Imperial vehicles were all built to a common specification over a period of centuries, so all Roman wagons and chariots were made with the same wheel spacing (or gauge).
OK, but WHY 4'8½"?! Because Chariots and Wagons were pulled by two horses, and so wagons and chariots were designed to be no wider than the posteriors of two horses, side by side. It made no sense to make these vehicles any narrower than this dimension so, railroad trains run on tracks the width of two horses backsides and it has been ever so - at least for the last two millennia!
