So, the only thing more important than a horse’s ass is two horse’s asses.
Know why Julius Caesar's book, "The Conquest of Gaul," makes no mention of chariots? Because they were obsolete as instruments of war before the Roman Republic was founded. The Romans had little use for them apart from parades and chariot races, which were a major form of entertainment for almost 1000 years.
There might have been ruts left in Britain from all the hand-drawn carts the legions were so fond of but it is unlikely there were any chariots there at all, much less in such number that they left a permanent imprint on the landscape.
And America once had more than 20 'standard' railroad gauges. Most railroads originally were built by venture capitalists, each of whom used whatever gauge he wanted. There was no national movement to standardize them until the industrial support of the War Between the States highlighted the folly of not having a single standard. The first intercontinental railroad used the 4' 8-1/2" gauge but the railroad industry itself didn't adopt the same gauge until 18 years later. The width of wagon ruts in England had absolutely no bearing on the selection. In fact there still are at least half a dozen "narrow" gauges in use across America today.
And Thiokol didn't manufacture the entire SRB on its own. It outsourced some components to other aerospace mfgrs (Pratt & Whitney, el al) and the pieces parts were built all over the fruited plain and shipped to Cape Canaveral for final assembly.
NASA states that the SRBs were 12.17 feet in diameter, which doesn't seem to have been much limited by 4' 8-1/2", probably because it wasn't.