Tuesday, September 18, 2012

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.

4 comments:

  1. My blog thing is indicating you have a post titled feedback loops, but it appears to be missing? Know whats going on?

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  2. Wouldn't the High Line be due for a pretty substantial overhaul in the forseeable future? They could redo how it crosses the NEC to allow for 4 approach tracks. I know the 25th street elevated trestle is in pretty bad shape.

    Also, it looks like there were once 8 tracks over university ave, and the southernmost bridge is disused. The rebuilt highline could renovate that bridge to make more room for passenger tracks/approaches.

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  3. James--Scheduling bug. Stay tuned, it's in the pipeline.

    Anon--Sure, that can happen, but it's better to plan for the existence of known constraints than to assume those constraints won't be there in the future.

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    1. Interesting, it still shows up in the feed

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