we’ve always done things this way

You will probably be familiar with the phrase ‘in a horse’s arse’, which although uncouth expresses disbelief or outright incredulity to great effect. For example, if I were to suggest that the Roman Empire had a negative influence on twentieth-century space exploration, you might reply in just this manner.

But before you do, consider this apparently trivial question: why is the standard US railroad gauge 4 feet 8½ inches? This does seem an arbitrary value. Or, to put it more colloquially, which horse’s arse came up with precisely that figure (quite a versatile metaphor, this one)?

If you’ve ever asked someone why they do something in a particular way that to you appears to be illogical, if not downright ridiculous, you’ll probably be familiar with the age-old excuse: “we’ve always done things this way.” And American railroads have always been built to this gauge. But why should this be the case? You should not be surprised to learn that the first American railroads were built by a handful of English expatriates, and that’s the way they’d always built railways in England.

So why did the English build their railways like that? It turns out that the first railway lines in England were built by the same people who built the pre-railway wagonways, and that was the gauge they used.

Which begs another question: why did the people who built the first wagonways use this gauge? Because they used the same jigs and tools that they used to build conventional road-going wagons, which had always used that particular wheel spacing.

Then why did wagons have this specific, and very odd, wheel spacing? Well, if they’d tried to use any other spacing, the wagon wheels would have been broken by the wheel ruts on the many old and not well-maintained long-distance roads in England, because 4 feet 8½ inches was the spacing of those ruts.

At this point, you may be wondering who built these old roads in the first place. Imperial Rome built the first long-distance roads in Europe, including England, to enable its legions to move quickly to wherever they were needed. These same roads were still in use long after the Romans had departed. And the ruts in these roads?

Roman war chariots formed the initial ruts, which everyone else had to match or risk destroying their wagon wheels. The chariots were produced for an empire with a well-organized military capability, which meant that they were built to a fixed standard, which included the small matter of the wheel spacing. According to the original specifications for a Roman war chariot, they were to be made just wide enough to span the rear ends of two war horses, which turns out to have been exactly 4 feet 8½ inches, according to later Imperial measurements.

Which is where space exploration comes into the picture. When a Space Shuttle is sitting on its launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are the solid rocket boosters (SRBs), which are made by a company called Thiokol in a factory in Utah. The engineers who designed these SRBs would have preferred to make them bigger, but after they are manufactured they need to be transported by train from factory to launch site. And the line from the factory runs through a tunnel in the mountains. The SRBs need to be able to pass through this tunnel, which is only slightly wider than the railroad track. And this track, as you now know, is as wide as two horses’ backsides. In other words, a major design feature of what is one of the world’s most advanced transportation systems was determined over 2000 years ago by a simple multiple of the width of a horse’s behind.

And you probably thought a horse’s arse was something insignificant.