"Yes, the entire system branches, yards & spurs."
Foreseen outcome: Corporate bankruptcy, probable commitment of senior management to lunatic asylums. Electrification is capital-intensive, only pays for itself on lines with very high traffic density. (Ignoring special cases: tunnels, etc.) In @, the PRR electrified, at guess, under 5% of its total trackage. (Ball-park figure, guesstimated: somewhere between 600 and 700 track miles electrified on a system with 10,000 total route miles.)
To make a scenario out of this that is coherent enough to think profitably about, you'd have to stipulate something bizarre: something to eliminate the cost of electrification fixed plant, perhaps.
In the end, U.S. railroads chose dieselization rather than electrification. A diesel-electric locomotive is basically an electric locomotive, but with its own on-board generating plant: this was affordable in a way that stringing overhead over all the tracks wasn't. As a fantasy, we can pretend this didn't happen: maybe elves don't like diesel engines and were wiling to donate overhead wires to make them unnecessary. Then we can ask, what would the all-electric equivalent of PRR's actual diesel fleet have been. ... Probably a bit smaller in over-all numbers, I suppose, given that PRR electric locomotives tended to be more powerful than first generation diesel units. And some new designs: box cabs were inconvenient for switchers (as builders of diesel switchers realized by about 1930), so maybe some sort of end-cab or centre-cab would have replaced the B-1 as a mass-production electric switcher. But it's hard to be sure WHAT would happen in fantasy-land!
((I hope you are not offended by my tone. I like fantasy -- even railroad fantasy -- so if you want to pursue this idea, that's fine: I'd love to see art-work of the GG-2 and the 4-8-8-4 electric that was at least considered for Pittsburgh line electrification. My own, personal, preference in "alternative history" railroad scenarios is for minimal changes to the history of @: scenarios close enough to the real world that reasoned argument about them is possible. As a general, philosophical(*), point, I'd stress what I said at the beginning of my previous reply. We know how to think about "what if"s that aren't too extreme: thinking about them is like thinking about the (near) future, or about places we haven't seen, in @. But a "what if" that requires a huge change from @ -- suspension of the laws of economics, if not the laws of physics! -- is an unanswerable question: there are too many possibilities, if you are going to consider far-fetched possibilities!))
(( (*) Many professional, academic, philosophers have thought and written about how serious reasoning about counterfactual scenarios works. One, the late David Lewis, wrote a short book, "Counterfactuals" that opens with the statement "If kangaroos didn't have tails, they would topple." Lewis was also a railroad fan: he once told me that he had wanted to have, as cover art for one of his books, a painting of the Pacific-type steam locomotive that the Great Western Railway (U.K.) was planning in the mid 1940s, and might have built if Britain's railways hadn't been nationalized for another two years!))