Ordinary Roman carts were constructed to match the width of Imperial Roman war chariots because it was easier to follow the ruts in the road left by the war chariots. The chariots were sized to accommodate the width of two large war horses, which translates into our English measurement as a width of 4' 8.5". Roads throughout the vast Roman empire were built to this spec. When the legions of Rome marched into Britain, they constructed long distance imperial roads 4' 8.5" wide. When the English started building tramways, they used the same width so the same horse carriages could be used. And when they started building railways with horseless carriages, naturally the rails were 4' 8.5" wide. Imported laborers from the British Isles built the first railways in the Americas using the same tools and jigs they were used to. Fast forward to the US Space shuttle, which is built in parts around the country and assembled in Florida. Because the two large solid fuel rocket engines on the side of the launch Shuttle were sent by railroad from Utah, and that line transversed a tunnel not much wider than the standard track, the rockets themselves could not be much wider than 4' 8.5." As one wag concluded: "So, a major Space Shuttle design feature of what is arguably the world's most advanced transportation system was determined over two thousand years ago by the width of two horses' arse." More or less, this is how technology constrains itself over time.