## Railway gauges

Episode 3 in a series “Things that are the way they are because of constraints that no longer apply” (or: why we don’t change processes we have invested in that don’t make sense any more)

The standard railway gauge (that is, the distance between train rails) for over half of the world’s railways (including the USA and UK)  is 4′ 8.5″, or 1.435m. While a few other railway gauges are in common use, including, to my surprise, in Ireland, where the gauge is 5′ 3″, or 1.6m. If you’re like me, you’ve wondered where these strange numbers came from.

Your first guess might be that, similar to the QWERTY keyboard, it comes from the inventor of the first train, or the first successful commercial railway, and that there was simply no good reason to change it once the investment had been made in thbat first venture, in the interests of interoperability. There is some truth to this, as railways were first used in coal mines to extract coal by horse-drawn carriages, and in the English coal mines of the North East, the “standard” gauge of 4′ 8″ was used. When George Stephenson started his seminal work on the development of the first commercial railway and the invention of the Stephenson Rocket steam locomotive, his experience from the English coal mines led him to adopt this gauge of 4′ 8″. To allow for some wiggle room so that the train and carriages could more easily go around bends, he increased the gauge to 4′ 8.5″.

But why was the standard gauge for horse-drawn carriages 4′ 8″? The first horse-drawn trams used the same gauge, and all of their tools were calibrated for that width. That’s because most wagons, built with the same tools, had that gauge at the time. But where did it come from in the first place? One popular theory, which I like even if Snopes says it’s probably false, is that the gauge was the standard width of horse-drawn carriages all the way back to Roman times. The 4′ 8.5″ gauge roughly matches the width required to comfortably accommodate a horse pulling a carriage, and has persisted well beyond the end of that constraint.