I was gifted a 9 inch pullman loaf pan and after many attempts, I finally filled the corners!
From the Wikipedia page:
European breadmakers began using square lidded pans in the early 19th century to minimize crust. Railway service pioneer George Pullman chose the loaf for use on his Pullman railcars for efficiency reasons.[2] Three Pullman loaves occupied the same space as two standard round-topped loaves, thus maximizing the use of space in the small Pullman kitchen.[3]