Why can’t booster rockets be wider than two horses?

On their journey from the factory in Utah to the launch site, rockets have to fit through a railroad tunnel that’s hardly wider than the railroad tracks. The tracks, in turn, are laid at a standard width of 4 feet, 8.5 inches, which American builders copied from tracks in England. Why this specific width? Because English train tracks were based on the ruts in English roads, and the first roads were built by Imperial Rome. Thus today’s rockets can’t be wider than a chariot was in ancient Rome: the width of two horses’ behinds.