30th St. Footnote: Hospital Interlocking

In the main post on this matter, I mentioned that the actual pinch point on 30th St.'s southern approach occurs where it passes under the High Line, as what were clearly the SB approach tracks (let's call them 3 and 4) pinch down from two to one to accommodate a large bridge pier where Track 4 should nominally go. I mentioned that I did not believe this pinch point was an issue, since it was clearly in place during the 1930s and '40s, when more passenger rail was operated than any other time in our history, and does not appear to have ever negatively impacted 30th Street Station operations. So creative dispatching can deal with a three-track approach.

The footnote is that if the inclination to apply concrete to simplify dispatching is pursued, then there is a simple way to ensure a four-track approach at all times into the station: duck Track 4 under Track 3. As (what was) the interlocking between the SEPTA and Amtrak 30th St. approaches is immediately west, or railroad south, of the High Line pinch point (see here), Track 3 can sort at the east end, and Track 4 at the west end, of this area^.

University Ave. is where it gets complicated. As it turns out, there are only four available passenger tracks across it, and split onto two different girder structures to boot. It's fairly apparent the PRR originally had at least an at-grade interlocking to sort here (a tower is still in place), but Amtrak and SEPTA, through agency fiefdom, have completely disassembled it, each crossing University Ave. on its own girder.

Fixing this so that Tracks 1-2-3-4 are N-N-S-S at University Ave.--important for implementing the proposed Chester Branch interlocking, not far to the west--will require agency cooperation as well as figuring out how the SEPTA NB track can fly over Amtrak's and still wind up at ground level to service University City station. This will be easier if we can replace the girder so that it has five tracks of space to work with, but let us try to deal with current site congestion to create "Hospital" interlocking.

This map shows how Hospital works assuming a Track 4 duckunder: Track 3 NB of Hospital becomes Track 4 SB of it, and the station throat interlocking would need to take it into account. Accepting the pinch point makes it far easier, as Track 4 can simply diverge railroad south of the Track 5 flyover (West Chester Branch EB). In either case, however, the need to end the incline at University City's railroad-west end will likely require the 30th St. approach tracks to depress slightly to make clearance; this depression will translate into a lengthening of the Track 3/4 incline between South and Walnut, with its technical limit station throat limits^.

Hospital Interlocking is the east half of a pair of interlockings intended to sort between the NEC, Chester Branch, and West Chester Branch railroad south, and 30th St.'s two approaches railroad north. The freight High Line/Chester Branch interlocking does not interact with these two in any way, and so will not be considered aside from noting that Chester Branch tracks 1 and 2 are 20 feet directly beneath two* CSX tracks.

Edited to add: One of my concerns about the Track 5 flyover was the lack of distance between the end of the University City station and the NEC main (Tracks 2 and 3). A quick check revealed, however, that there is 500 feet distance between those two points, which allows a flyover to just make an 18-foot girder clearance, 20 ft track clearance, at a 4% grade.
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^ Addendum 9/20: Another possibility that occurred to me after I completed this post is to simply run Track 4 on the surface and Track 3 underneath it. That does away with the excessively complicated wayfinding necessary in what I had first described.
* The High Line has the capacity to be a two-track main; the second track is disused. This exercise assumes that freight traffic growth allows NS and CSX to rebuild the second track and CSX to run a two-track line from its main to it.

30th St. Approach Capacity?

Professional transportation engineer Liam left me this comment on my previous post, The Alternative to the Market East Tunnel:

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.

I think it's a manageable constraint, but there's definitely more going on here than just Zoo.

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.


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.

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* 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.