“Because every inch counted in a Pullman [dining car] kitchen, storage space was critical. It was no surprise that Pullman was delighted by the shape of a loaf of Pain de mie bread. It is a sturdy symmetrical four-square loaf. To accomplish this physically constrained, tanned brick of delight, eighteenth century Boulangers placed the risen dough in a rectangular metal box fitted with a sliding lid for its second rise. The dough could only grow to the dimensions of its box and therefore produced an even, slightly dense interior. It was bread perfection — at least, geometric perfection. Other, unrestrained, loaves of bread when baked freely rose in the pan into a happy domed top. Not only was the flat-topped, pain de mie stackable, to Pullman’s delight, four could be stored in the same space as three precarious round topped loaves.”

https://www.thecookscook.com/article/the-pullman-loaf-legacy/