[kanox]

> 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.