The international standard railway gauge (distance between the rails) is 143.5cm (or a smidgen under 4 feet 8.5 inches for the metrically challenged). A strikingly odd width but is, nevertheless, the standard gauge. Why? Because that's the way they built them in Britain, and the Brits built the railways in the US, Japan and many other countries around the world as they desperately tried to revive the expansion of their crumbling empire.
The gauge is a legacy of the earlier tramways and railway engineers used the same tramway jigs and tools for building the railway wagons. The tram coaches in turn, had been built using the standard jigs used to build horse-drawn stage-coaches.
Horse-drawn carts had that wheel spacing because it was the standard spacing of the wheel ruts gouged in the roads. Long distance roads in Britain (and elsewhere in Europe), were first built by the Romans as they expanded their empire. The Romans had earlier experimented with the design of battle chariots and found that two horses, side-by-side, was the best arrangement in terms of speed and maneuverability. Although their engines were only 2HP, we shouldn't under-estimate the importance of these early-day test-pilots. Their findings fixed the width of chariots, and hence ruts in the road.
Now let's shunt forward two thousand years to the design of the Space Shuttle,
You may have noticed there are two huge ancillary rockets on the sides of the main fuel tank. These are known as 'reusable solid rocket boosters' and are made by a killing-machine firm called ATK Thiokol in Utah. The assembly plant for these boosters is located on western side of the Rocky Mountains and to transport them to the Kennedy Space Center, rail is the cheapest and quickest method. The line from the factory runs through a tunnel so the booster rockets have to be made smaller than the tunnel. This tunnel is only a bit wider than the track, and therefore the rocket design had to take this into consideration.