https://magazine.engineerjobs.com/2015/3-engineering-urban-myths.htm High points:
The legions didn't use massive numbers of chariots. Even wagons weren't all that plentiful for the legions.
In the US early on there were a score of different gauges in use.
I did run across this interesting bit: https://www.zmescience.com/science/news-science/ancient-roman-road-repair-0423/
Roman road works were really sophisticated, and their maintenance was no different, as we’ve learned from a new study published in the American Journal of Archaeology. During a 2014 survey of Pompeii’s streets, researchers at the University of Massachusetts Amherst and the University of Texas at Austin were surprised when they consistently found iron between and below paving stones of the city’s most important streets.
Like other major towns, most of Pompeii’s streets were paved with stone. However, the passage of carts on a daily basis eroded the stones, forming deep holes and ruts which are still easily visible to this day. In fact, over a century and a half ago, the American satirist Mark Twain based his complaints about the corruption of city officials at Pompeii on these cavities:
Although the Romans were quite advanced in their public works (and corruption was indeed a problem in Roman society), Twain was perhaps not aware of how complicated road repair during that time could be. Repaving the street was out of the question in most situations — it was simply too expensive and time-consuming. If a narrow street was damaged, traffic could be blocked for months until specialists finished repaving the street.
Examples of iron fillings and drops on Pomepii’s ancient stone-paved streets. Credit: Eric Poehler.
In Pompeii, at least, the Romans devised a creative solution to their problem. The city’s engineers heated iron to a molten state, then poured the material onto, into, and below the eroded paving stones. Hundreds of individual street repairs were discovered thus far in the city.
The molten iron was poured alongside other filler materials such as stone, grounded terracotta, and ceramics. Once the metal cooled down, the whole mash solidified to completely fill and cover holes. According to the archaeologists, this method was much cheaper and quicker than repaving a street.
It’s not exactly clear how the Romans carried out such repairs, but the researchers have some clues. We know that the iron would have needed to heat to about 1,600ºC (2,912ºF), a temperature which Roman furnaces could accommodate. Iron drops were found on sections of the street that didn’t require repair, suggesting that it was accidentally spilled while being carried, a task likely reserved for slaves.
In the future, the researchers hope to analyze the chemical composition of the iron from the street filings to find its source. They would also like to survey more Pompeii streets.