Well at least it is a standardize system. If not
there could or would be systems of various sizes.
It would be a mess if there were different systems
across the USA, etc.
IIRC in the early days of American railroads there was no standard gauge and it was a mess. Most early trains came here from England so the gauge was standard for a while. Once we started making our own trains and railroads had a huge expansion almost every railroad had different gauges. Was so competitor’s trains couldn’t fit on their tracks. They eventually agreed on a standard which is the one we have today.
“It would be a mess if there were different systems
across the USA, etc.”
One of the problems in the south during the civil war was that there were a multitude of rail gages.It wasn’t unusual for rail companies to be small outfits that serviced a relatively small area.
Whoever built the stretch of railroad between, say, Richmond, Va and Lynchburg, Va would build it in whatever gage suited his needs and budget.
Freight would often be unloaded off of a narrow gage train and reloaded onto a standard gage train, slowing the delivery of freight by days or weeks.
In the north there were even more small rail companies but the rails were more standardized, simplifying the movement of freight and war supplies.