I think this is referring to the standard width of rail cars, which are required to move some component parts. Very, very generally, rail cars are the width th

I think this is referring to the standard width of rail cars, which are required to move some component parts. Very, very generally, rail cars are the width they are because they started in Europe, and they had to fit on roads, which were built to accommodate older cars, which had to fit in wagon wheel ruts, which had to fit in Roman chariot ruts. I'm not sure it's fact, but it's a common story. I heard it from Prof. Channing's ChemE lectures on Stanford's iTunes U.