Funny how far back conventions can go..
Eg Railway standard guage -- was developed due to horse coachbuilding conventions, length of axle determining width of modern railway tracks.
But why did horsedrawn carriages use an axle of that width?
The majority of roads in Europe at the time horse drawn coach services became prominent, were original or adapted Roman roads.
So the width of the track on modern high speed rail services was determined by some Roman engineer over 2000 years ago. Where he got the idea from, no-one knows.