Alaska Airlines is retiring four unique 737-400s that are half-freighter, half-passenger and is replacing them with the world's first all-freight 737s.
Alaska has a special situation; only a small part of the state is served by rail, road or sea freight, and many areas rely entirely on air deliveries. That's how Alaska ended up with its 'combi' planes, with a passenger compartment at the rear and the front cabin turned into a freight hold. Note the missing windows at the front above.
Those planes are now retiring, and being replaced by 3 737-700s that have been converted from passenger to freight service. Both the 'combis' and the new all-freight planes deliver bulk cargo to distant towns that serve as hubs from which smaller planes and other means spread it out into even more remote areas.