In North America - in fact for most of the world - standard railroad track has 4' 8-1/2" gauge {the inner distance between the two rails}. According to legend, this is because British wagons were of that width because they fit in ruts in Roman roads.
In some cases, US railroads are built to a narrower gauge {for example, plantation locomotives .... the locomotive originally used on Hawaiian plantations was built with 3' gauge}
Hawaiian Steam Locomotive in Connecticut - PentaxForums.com
The East Broad Top tracks - which were originally built to haul coal through the mountains of Pennsylvania - were also built to 3' gauge. After it was no longer needed to haul coal, the EBT was sold for scrap - but the new owner decided it actually had value carrying tourists. I took this picture in 1986. A few years later it was shut down .... as far as I know, docents still give occasional tours through their extensive shops, but no one has gotten around to finishing the job of scrapping it.