Because they were built in Eastern Europe and western Asia. Vtheir horses and roads were descendent from the Mongols.
In modern times they retained the gauge so that invaders from the west could not use their equipment to supply their forces.
I am reminded of the story I used to tell in my consulting days.
A woman was cooking Sunday dinner. She took the Roast Beef and cut off an inch on each end before putting it in the oven.
Her daughter asked her why. The mom responded that it was how she was taught by her mother. The next Sunday, the daughter asked her grandmother why she cut the ends off from the roast. She said that her mother taught he that way.
So, they got on the phone to call the girls great grandmother to ask her why she cut the ends off the roast.
She laughed about it for a minute. Then she told them that when she was first married, they were poor. So they only had one pan, and the Sunday roast was too big for the pan, so she cut off a little bit so it would fit.
The moral of the story is that in a lot of processes it is important to understand why they are done that way—because you will find the reasons don’t exist any more.
As an aside, we also found that if our employees just followed the rules as they were originally written, things would work fine.
Kind of like the Constitution.
The 4’ -8 1/2” “Standard Gauge” of railroad track is “standard” only in some countries, such as the U.S. Only about 2/3 of all rail line uses that spacing between rails. Gauge typically varies between 18” and 5’ -6”. Industrial lines where space was an issue, like mining and even trench systems constructed during WWI, had 12” or 18” track. Many railroads in New England and places like Colorado were 24”, 30” or 36” gauge, called “narrow gauge.” BART lines today are wider, at 5’ -6” between rails.
When George Stephenson started building rail systems in England he favored 4’ -8”, but 1/2” was added to allow play on curves. At that time 5’ -3” was “standard” in Ireland, and 7’ -1/4” was used by Stephenson's rival the Great Western railway. Stephenson won out.