What would be your response to criticisms that you're overlooking capacity issues at 30th Street? There are only two tracks headed south from the station, after all; the line doesn't open up to 4 until the Regional Rail tracks come in from University City.This is true. Agency turf wars are a continuing problem, and have resulted, more than once, in extraneous concrete dedicated to eliminating versatility and redundancy previously built into the system--Zoo is a good example of this, where no fewer than half of the paths through the interlocking have been removed. And when the Market East tunnel was first proposed, a popular theory was that Amtrak didn't want to to have to share space with SEPTA anymore (never mind the fact they have different concourses, and different platforms with different approaches)...To put it another way, the Market East tunnel, like lobotomizing Zoo, is a concrete solution to an organizational problem.
I think it's a manageable constraint, but there's definitely more going on here than just Zoo.
Now we move to the real meat of Liam's specific question. There are two questions embedded within this query: (1) how do we increase the capacity of 30th St.'s south approach, and (2) (what he assumes the answer to is yes) does the 30th St. approach present a capacity problem?
The answer to (1) is that only 2/3rds of the approach's capacity, at its narrowest point, is currently in use. The ROW pinch occurs under the High Line overpass right before the Amtrak and SEPTA approaches meet; under this overpass there is room for three tracks, two of which are currently in use. Those two were originally the northbound approach; the southbound approaches have been converted into two spur tracks. Originally, they merged just before the High Line underpass and would have merged again where the upper- and lower-level approaches met, resulting in four passenger-dedicated tracks at the University Ave. overpass, tracks which would then sort into NEC tracks 1-4 and the two West Chester Branch ones.
So we can assume that--if full capacity can be reactivated--30th St.'s south lower-level approach is (technically) three tracks at its greatest pinch, and with smart dispatching (effectively) four tracks. Plenty of capacity, especially since through trains really only need four or six tracks (three platforms), half the station's current capacity (five platforms, ten tracks).
Of greater concern is the north approach, which runs straight into one of the most complex pieces of rail infrastructure in the United States: Zoo Interlocking. Remember in this discussion that the historical owner of this infrastructure, the Pennsylvania Railroad, maintained two different paths for any possible movement through the interlocking, and that there are six feeds into it (Main Line, Northeast Corridor, Reading interchange, High Line, Upper Station Approach, Lower Station Approach). For our purposes, that means the PRR maintained a four-track north lower approach, only half of which is now in use*--this is what I mean when I say Zoo has been lobotomized.
The simple solution of reconnecting several switches, and putting the Schuylkill River bridge's fifth track back in, grows the capacity of this, the busier side of the station.
Now we can address the second question. Passenger rail growth will demand capacity improvements, indubitably, but 30th Street Station was one of the last prewar passenger rail termini built in the United States, and as such, was engineered with a far greater capacity than e.g. the original layouts of any of the stations Broad Street Station was built to replace. Its approaches were state-of-the-art and able to handle 1940s-scale rail traffic without a hitch; it's very unlikely that the approach tracks within the existing PRR footprint (not all of which, remember, are currently in use) will hit "concrete" capacity limits even in the most optimistic future traffic projections (such as those Corey Best produces).
That said, as with any finite structure, there certainly is a capacity ceiling--but the traffic scale one would expect near it, which we are currently far from--would be able to justify even the most futuristic infrastructure improvements--much as the Chuo Shinkansen has become viable largely because traffic demand between Osaka and Tokyo has surpassed every capacity ceiling the Tokaido Shinkansen has to offer. Then, and only then, would a Market East tunnel become feasible.
* The current situation offers two possible southbound approach tracks, but only one northbound one, through the interlocking. Only one of the southbound approaches, however, shows high-grade maintenance.