It’s nothing more than a straight-sided loaf pan with a lid on it. It yields a loaf of bread with perfectly flat sides all the way around and perfectly square corners. What good is that you say? Well granted it’s an aesthetic thing mostly. A perfectly square loaf — as seen from the ends — yields perfectly square pieces of sandwich bread and pieces of toast. That’s great for us uptight types who like our foods neat.
However baking in a Pullman pan is also functionally different from baking in an open-topped loaf pan. In an open-topped pan the bread can expand freely. Not so in a Pullman pan, at least one that’s been loaded properly. There the rise of the loaf is constrained, not a lot but a little, and that tends to keep large bubbles from forming, so the crumb of the bread is fine and tight.
Just another aesthetic thing? No not really, as sandwich bread with big holes in it can be perilous for the sandwich eater, especially if he’s wearing an expensive tie. Tight holes keep condiments in and soak up melted butter, which make Pullman sandwiches and toasts more civilized experiences with a smaller dry cleaning bills. And really the eating experience is different too since texture effects mouthfeel and flavor.
And speaking of texture, the main reason Pullman pans were first invented in Europe was to do away with crust. I know what you’re thinking: quelle horreur! There were once Europeans who baked bread without crust? On purpose?? Indeed there were, and still are. Think of Pullman loaves like pain de mie as a special-purpose bread where crust isn’t welcome.
Again, consider sandwiches. Lots of people like to try to dress up sandwiches by using hearty country loaves or baguettes. It’s a nice idea, but try biting through one of those. You end up clamping down on one end of the sandwich with your teeth and pulling with both hands on the other. When the portion in your hands suddenly comes loose the fillings go flying in the opposite direction, onto your dining partner and we’ve got cleaning bills again.
No, a pleasant eat-able sandwich is soft all the way around. This is what the Euros were thinking when they invented the lidded loaf pan about 150 years ago.