This picture shows a solid structure within the wheel, although they did develop spoked wheels with metal tyres and axle sleeves.
The US rail system is also based on the Roman Chariot spec and is 4ft 8 1/2 inches. As the Romans spread throughout Europe and built roads most of the ruts that appeared were from Chariots. From this all other road using wagons would build to the same gauge in order to stay within the same ruts.
The US rail structure was built by English Ex-pats who brought with them patterns and Jigs used in the Tram systems in England at the time which were based on the Romans Disciplines.
I am also amazed that the Romans seemed to have invented the thong along the way to ..
Bigger picture of same chariot