The U.S. standard railroad gauge (distance between rails) is four feet, eight-and-one-half inches.
Why such an odd number? Because that’s the way they built them in England, and American railroads were built by British expatriates.
Why did the English adopt that particular gauge? Because the people who built the pre-railroad tramways used that gauge.
They in turn were locked into that gauge because the people who built tramways used the same standards and tools they had used for building wagons, which were set on a gauge of four feet, eight-and-one-half inches.
Why were wagons built to that scale? Because with any other size, the wheels did not match the old wheel ruts on the roads.
So who built these old rutted roads?
The first long-distance highways in Europe were built by Imperial Rome for the benefit of their legions. The roads have been in use ever
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