This is similar to why booster rockets are a certain width — they needed to be moved by trains that went through tunnels carved by the Romans. Those tunnels were the width of two horses pulling a chariot.
Romans didn't use chariots, except for chariot racing. For military purposes, they used cavalry and footsoldiers. For domestic transport, they used wagons. And as far as I'm aware, they never constructed "tunnels" for transport - are you referring to mine tunnels?
GP is probably referring to the standard railroad gauge which is alleged [1] to have been derived from the width of roman chariot wheels and which would have driven the train tunnel width in modern times.
