The US standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches. Why was that gauge used? Because that’s the way they built them in England, and English expatriates built the US Railroads.
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’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.
Why did the wagons have that particular odd wheel spacing? Well, if they tried to use any other spacing, the wagon wheels would break on some of the old, long distance roads in England, because that’s the spacing of the wheel ruts.
So who built those old rutted roads? Imperial Rome built the first long distance roads in Europe (and England) for their legions. The roads have been used ever since. And the ruts in the roads? Roman war chariots formed the initial ruts, which everyone else had to match for fear of destroying their wagon wheels. And Imperial Roman war chariots were made just wide enough to accommodate the back ends of two war horses.
Now the Space Shuttle had two solid rocket boosters, or SRBs, attached to its side. These SRBs had to be shipped by train from the factory to the launch site, and so had to fit through that railway tunnels on the track. The tunnel is slightly wider than the railroad track.
So, a major design feature of what is arguably the world’s most advanced transportation system was determined over two thousand years ago by the width of a horse’s backside.
Quoted from Grove Booklets email 30/11/2003