Basically there are loads of different railway gauges around the world, and indeed, inside the USA itself, which historically had no single standard size.
All roads, railways, and thus tunnels and bridges everywhere are very approximately multiples of the width of a horse's pectoral/pelvic girdles. Nothing especially Roman or anything else about it.
https://www.snopes.com/fact-check/railroad-gauge-chariots/
Basically there are loads of different railway gauges around the world, and indeed, inside the USA itself, which historically had no single standard size.
All roads, railways, and thus tunnels and bridges everywhere are very approximately multiples of the width of a horse's pectoral/pelvic girdles. Nothing especially Roman or anything else about it.