It's a fun myth but it's not true. The SRBs for the Space Shuttle were designed to satisfy their mission criteria. While any similarity between US rail widths today and Roman roads is due more to happenstance than a direct result.

https://www.snopes.com/fact-check/railroad-gauge-chariots/

> What's True: The standard U.S. railroad gauge is similar in width to the wheel spacing of Roman chariots.

> What's False: That similarity is based much more on coincidence and inherent physical limitations than a direct line of imitation.

The width of chariots/wagons/car/trains doesn't vary all that much, not compared to stuff like sailing ships.

But the Space Shuttle SRBs have a considerably larger diameter (3.7m compared to 1.435m rails) so it's more likely due to civil engineering standards: you can only transport something so big until you need to take down power lines and cut down trees.

The Falcon 9 has the same diameter and it was explicitly chosen because it's the maximum transportable by road.

Um, nope. The choice of railroad gauge was a historical artifact: in the beginning, everyone had a different gauge, then we had like 15 gauge standards and "standards", and one of them eventually prevailed (through British administrative fiat). The choice was not due to an inherent superiority of this particular gauge, but due to a campaign by George Stephenson. From there, it was mostly network effect: popularity breeds popularity.

Gauge is, most of all, a tradeoff between construction costs (tunnels, bridges, cuttings, oh my! Every additional inch of the gauge gets real expensive in Actual Terrain; ditto for train stations: narrow gauge can fit many more tracks next to each other), and between operating costs (wider gauge cars can be wider _and_ higher, as they're inherently more stable; thus, more cargo on same number of cars).

( ObXkcd: https://xkcd.com/927/ )

On another note, human intelligence is limited by the width of the birth canal.