Why would that be? The nearest thing to a global standard rail gauge is standard gauge and it isn't a simple number in either SI, 1.435 m, or US Customary unit

Why would that be? The nearest thing to a global standard rail gauge is standard gauge and it isn't a simple number in either SI, 1.435 m, or US Customary units: 4 ft 8 1⁄2 in (see https://en.wikipedia.org/wiki/Standard-gauge_railway https://en.wikipedia.org/wiki/Standard-gauge_railway). There might already be a standard of some kind that could be used either directly or as a sub-multiple the same way that European kitchen fitments are generally 600 mm wide modules. There is nothing magical about units of measurement, neither SI nor what Americans persist in calling Imperial (when they mean US Customary or even worse US Survey Foot).