FYI, to everyone talking about four tracks, remember, Dave just thinks he's modeling the Middle Division. In reality, like so many other wise Pennsy modelers he'll realize he's really a Buffalo Line guy 
Haha, LOL. For context for everyone else, there's a theory that ever Pennsy modeler eventually models the Buffalo Line.
The combination of heavy traffic on just two tracks through mountain scenery and Middle Division-quality infrastructure (plus some cool scenes like Sunbury, Norry, and Renovo) makes it much more model-able than the Middle Division while keeping the heavy-duty railroading that makes Pennsy appealing...just scaled to more manageable levels. All your favorites were still there... M1s, I1s, L1s, Sharks, F-units, passenger and mail trains, and so forth.
I may go a different direction in the future and just do vignettes...some prototype scenes, some "generic." There are two HO scale model railroads based on Pennsy at the Railroad Museum of Pennsylvania. One's a relatively faithful model of the Middle Division including Lewistown, Granville Bridge, Mifflin, and the Denholm Coal Wharf. The other is a vignettes-style layout with "almost Philadelphia," "almost Lancaster," and "almost Mount Union."
The latter appeals to me for the very reason that I want to model more than one section of the Pennsy. I love the urban electrified stuff. I love the Amish country stuff. I love the Middle Division running. And I love the area around Altoona. So much to choose from, but limited space, time, and money. So vignettes may be the answer.