Riffing off of what Rear Adm. Briem (Ret.) had to say about what Diana Nelson Jones had to say about the T today, I think it's interesting to consider the map that Chris posts:
OK, maybe half a map. Moving on...
Now, I don't like being a technological determinist (I think of it as lazy sociology), but on some level I'm a materialist and I believe that our societal condition is reflective of real, tangible things and not just some aethereal Hegelian Thesis. So, I will readily accept that things like the automobile, highways, two car garages, etc. changed the way we approached urban planning. If there's any doubt as to the impact of the automobile on the development of Western Pennsylvania, ask yourself if places like Cranberry, Monroeville, or Robinson Township would even begin to approach their current scale without it.
So, let's step back and consider, for a moment, how the urban fabric would be different if the above map had actually played out. Let's assume, therefore, that by 1985, the entire rapid rail system would have been built, stretching from nearly Armstrong County to the Greater Pittsburgh Airport.
If you read the transportation plan you'll notice two things immediately. First, while it notes that Pittsburgh's population has dipped slightly and is trending downwards, it predicts that the population will move out to the suburbs and the county trend will be a 50% increase in population over the next 15 years. Second, the study believe that the current trends in employment will remain more or less constant all through 1985.
(You may take a good hearty laugh right now.)
OK, so that didn't work out too well, but the planners of the day didn't know that yet. It does note, however, that there's a trend towards personal automobile ownership, but I'm not quite sure it grasped the full magnitude of what was about to happen.
Now, the map above is a little blurry, but there are 8 major corridors: Ohio River, North Hills, Allegheny Valley, Wilkinsburg-East Hills, Monongahela Valley (really the lower East End), Pleasant Hills (really the Mon Valley), South Hills, and the West End. All of these corridors focus in on Downtown, in the wheel & spoke model that Pittsburgh's bus riders are familiar with.
Interestingly, the report suggests that the Downtown to Monroeville Transit Route would be the most used, followed by a route to Route 19 and Castle Shannon, while more people would head towards the County Airport than Greater Pittsburgh International Airport. Highest priorities, then, were the route from Downtown to Monroeville, from the North Hills to South Hills, then to the County Airport.
I guess in some sense, the report was generally correct: the Monroeville Route is pretty much the existing East Busway and the Castle Shannon Route is pretty much the existing T-Line.
But what was not really predicted was the expansion Northward and Westward. The report notes that the demand to go to the Airport was minimal at best. Also, Oakland is presented almost as an afterthought to the planning, with so-called "knowledge economy" still being years after the impending collapse of the steel industry.
Also not taken into consideration is the racial migration and segregation patterns that fully took hold in the '60s. The Civic Arena had already been built, but the Middle and Upper Hill still had some life left in them.
What would a rapid transit enabled City of Pittsburgh look like then?
I think in some sense it would, from a distance, look much the same: the patterns of development (Robinson and Cranberry excepted) are served mostly by the East Busway, T-Line, and North Hills HOV Lane. The creation of a single rapid transit system would have probably caused those areas, particularly Monroeville, to concentrate into more densely organized enclaves to take advantage of the proximity of rail stations. Ergo, I believe that the Miracle Miles Shopping Center would have been more of a Miracle Acre. If you take a look at the development in the South Hills where the T did in fact go, you see fairly concentrated development (although this could also be explained by the tight topography of the area), which sort of backs up this suspicion.
If a line had gone out West towards the airport, however, we would have seen a much different pattern than what we have now. Robinson and South Fayette would not have slouched all over the airport area and people would stop complaining about commuting into Downtown through the Fort Pitt tunnel, because of all the people coming in from Robinson and South Fayette. It might have actually been a halfway decent place to live.
I also imagine that places like East Liberty and Homewood would have not suffered as sharp a decline over the last 40+ years, as these difficult-to-access neighborhoods would have been tied into a larger transit network capable of disbursing residents throughout the county. Perhaps that might have convinced transportation planners that stupid roads like the Mon-Fayette Expressway wouldn't really be needed.
Downtown and the surrounding areas would have probably benefited from the cheap, quick transportation and companies that would eventually seek new office space would seek to be if not in or near downtown then along one of the transit arteries. If anything, the oil crunch in the '70s would have made such sites much more valuable.
This assumes that the population trends remained the same and that by the mid 1980s, with the collapse of the Steel Industry, the City fathers didn't just throw up their hands and give up on the lines (which is some sense, they did). Not addressed in the plan, of course, are former industry site like the South Side and Hazelwood, one of which, today, is booming while the other remains stagnant. Because it would have been less accessible, I could see that the South Side may have lost it's trendy character to areas like Uptown, which would have been easier to access by the student populations.
Anyway, this is all just a big old thought experiment which needs more than just three glasses of cheap red wine to fully hash out. Planning for such a large project with a large time horizon is hearly impossible. In reality we're kinda stuck with what we got.
Until the Port Authority develops time travel.