Zoo Interlocking is the junction between the Keystone Corridor and Northeast Corridor and one of the most complex interlockings in the nation, shifting passenger trains from two distinct main lines to two distinct station throats, and freight trains three ways.
In the past it was even more complex, with a fourth major direction for freight as well as two different paths for each major movement (that is, two different ways to get from the Northeast Corridor to Suburban Station, two different ways to get from the Main Line to the lower level of 30th St. Station, two different freight bypasses of 30th St. Station, etc.) Much of this redundancy has been stripped out by warring organizations over the years, leading to massive backups when any part of this intricate network fails.
However, Zoo has a key failure: although two of the main wye's three branches are able to handle high-speed interlocking, the third is not. This is not because of a failure of interlocking design, per se, but rather because the best engineering available at the time the interlocking was initially set up forced the third direction to run through a significantly tighter-radius curve than the other two.
As you may have guessed, the tightest-radius connection is the Northeast Corridor side, the east side closest to the zoo Zoo was named after. This connection was built the way it was because it had to simultaneously do two things: (1) elevate from the freight yards along the Schuylkill floodplain--now 30th St. Station, its two leads, and the Powelton Coach Yards--to the level of the river crossing, about 75 feet above the waterline, in only about a mile, while at the same time (2) accommodating the junction between this route and the Pennsylvania's mainline about halfway between.
No matter why it was built, however, it is obsolete if one wants to bring 200 mph trains into 30th Street Station. Since I have it on good authority that that atrocious tunnel proposal is dead even at Amtrak (and was always a consultant's dream to begin with), we have to look at smaller-scale (and cheaper) interventions to accomplish what we want.
Assuming an Airport routing is still desirable--it is certainly feasible--three major interventions are necessary to convert the NEC to a 200 mph + (250 kph +) route through Philadelphia County. They are:
1) Bypass Frankford Jct., which has to be traversed at ~75 mph,
2) Bypass Zoo Interlocking into 30th St. Station's north lead, and
3) Convert the former Reading's Chester Branch into an Airport routing.
See map here.
Bypassing Frankford Jct. is technically the easiest, and the least expensive: the path of Torresdale Avenue, which turns into Erie Avenue at Kensington Avenue is a natural bypass route, and a couple of takings in industrial properties--possibly little more than air-rights takings, as the HSR main would likely have to clear at least one local track before rejoining its own track--would be all the property acquisition needed. Takings would be exclusively industrial.
The trains can traverse Erie (Torresdale) either via an aerial structure or a cut-and-cover tunnel. All types of tunnels are somewhat more expensive than bridges; however, the aerial would have to clear the El at Erie-Torresdale Station, meaning it would have to be ~50 feet off the ground at its apex, rendering it somewhat more expensive than it would otherwise be (e.g. if the aerial could remain at ~30 feet off the ground). So both modes approach cost parity.
Since you can recruit DOT engineers to design an aerial (it's essentially a highway exit overpass, but for trains) whereas even the most basic cut-and-cover tunnel will likely require weaving through underground infrastructure, and since I prefer bridges over tunnels, I'll go with an aerial in this instance. Price tag's probably $250 million, once value-engineered.
Technically the most complex is the Airport routing. Its complexity has nothing to do with attempting an inner-city greenfield routing--it has everything to do with rail uses along the routing. Other than SEPTA's Airport Line, the principal use of the current line is a freight routing to (unsurprisingly enough) Marcus Hook. It is also the routing closest to the river, and thus the one with the most port-related spurs and traffic.
In order to close the Chester Branch east of Essington, CSX will almost certainly require a bypass. My proposed routing, the "Hog Island Freight Bypass", would utilize the former Pennsylvania R.R. 60th Street Branch, most of which is intact (ROW unused for freight is currently used for Auto Mall junk car storage). However, unlike the historical 60th St. Branch, the Hog Island Bypass would meet the CSX main through the former Brill property in Southwest Philly. The connection between the 60th St. Branch and the Chester Branch built for the Airport Line would remain in use as a freight spur for any traffic generated along this alignment (though I don't remember Eastwick's industries having freight rail access).
Now that we've dealt with the freight rail, the next step is to provide the optimal ROW for the passenger service. The Airport Line currently uses the 60th St. Branch to the Chester Branch, but this forces the train through a pair of slow and inefficient corners; a true high-speed routing would continue along the dead-straight Chester Branch until it meets the Northeast Corridor.
The problem with this is that this routing has a surface interchange with the CSX line across the Schuylkill; in addition, the Chester Branch between this interchange and the NEC junction is currently used by CSX as its High Line access line. However, because we moved the freight route for Marcus Hook trains from the Chester Branch to the Hog Island Cutoff, we can go ahead and run passenger trains under the High Line access line, coming out onto the Northeast Corridor. (This routing also allows Airport Line trains to stop at Bartram's Gardens.)
Next, we move to the Airport. The first look we have at the Airport is that we can add a new through cutoff between the Chester Branch and Airport termini, with a somewhat sharp curve by the station. However, this would likely be the more expensive option. A better proposal would be to instead have the through-train Airport station along the Chester Branch, due to the superior curvature (i.e. none). But this is on the wrong side of I-95. So, if this routing is the option decided upon, it will likely become necessary to convert the current Airport Line terminal access to an AirTrain-style light metro. (Fortunately, most of the infrastructure is in place for this; the primary expense would be acquisition of the proper equipment.)
While the Chester Branch has been grade-separated in Philadelphia, such is not the case in Essington, where the small town is hard against the rails. Grade separation, needless to say, is essential here; since we are close to the water line, this will likely take the form of an aerial about 20 feet above the existing freight line (i.e. Plate H double-stack clearance). As the fast passenger line being discussed would allow rerouting the Regional Rail line as well, this local will want a stop in downtown Essington--that is, where the line crosses Wanamaker Avenue. This aerial structure would thence continue across the mouth of Darby Creek and then parallel the Industrial Highway (on top of a disused rail line, possibly PRR, between the road and the Chester Branch) until it returns to the NEC, just south of Eddystone.
What kinds of expenses can be expected for such an undertaking?
1. New track installation (but no re-alignment) between the 60th St. Branch-Chester Branch junction, to the north, and the Penrose Avenue bridge (Penrose Ferry Road) to the south. This may need to be done with grade separation, as it appears to have historically been a grade junction. Assuming prior easement claims can be asserted... $300 million, most of it sunk into the grade separation at the Hog Island-Chester Branch junction.
2. Cut-and cover tunnel under the freight Chester Branch between the NEC and CSX mains... $500 million.
3. New east cutoff between Eastwick and Airport. / New Airport station, conversion of terminal network into AirTrain system... $300 million (whichever way).
4. 3-mile aerial structure from the 4th St.-Chester Branch level crossing west to new NEC junction... $700 million.
5. ~5 miles of new constant-tension electrification... $100 million.
All of these add up to an estimated price tag of... $1.9 billion. (This compares favorably with the asinine tunnel, whose initial cost estimate would be somewhere in the neighborhood of $8.5 billion and work its way up from there.)
It will cost at least $2 billion to bring HSR to PHI. This cost also includes a SEPTA expansion. (The cost of this expansion alone would be about half that, as it wouldn't need to worry about the extended aerial through Essington or grade-separating the junction between the 60th St. and Chester Branches, or restoration of the 60th St. Branch through the Auto Mall, but it would need to worry about a new alignment into the NEC on the east end, dealing with the Airport network, and extending electrification. And without the HSR line, the west-end return to the NEC isn't viable, making the line's natural terminus Essington.)
Station stops on this Airport Line would be: Bartram's Gardens, Elmwood (63rd St.), 70th St., Eastwick (84th St.), Airport (somewhat west of 90th and Bartram), Tinicum (4th Ave.), and Essington (Wanamaker Avenue). Service frequency would likely have to be half-hourly, due to the lack of slots on the SEPTA Main Line.
Let's return, then, to where we started: bypassing Zoo. Zoo sits between the Frankford Jct. bypass and Airport extension in terms of complexity and hence expense. One one hand, the Zoo bypass will be more complex than a Frankford Jct. bypass, due to the fact it has to deal with the Schuylkill; on the other, it has nowhere near as many moving parts as the Airport project.
Because of Lemon Hill, the only sensible place to diverge the bypass is where the NEC and Schuylkill Expressway split, just north of Spring Garden; a narrow easement opportunity is available between the Schuylkill and 34th St.
There are two ways of approaching this bypass: a bridge or a tunnel. Both have their challenges. The tunnel would be an immersed-tube (it's shallower) but the east approach would need to be drilled out of solid schist. On the other hand, the bridge would likely require a very steep west approach--it would have to clear the Schuylkill and the, er, Schuylkill--and it would also need a new easement through Fairmount Park. However, unlike the tunnel, the bridge would rejoin the existing line more quickly, and hence be quite a bit cheaper.
The tunnel offers superior engineering. The bridge offers advertising...but it would also attract NIMBYs who wouldn't want to change so much as a tree in the park.
My estimate is that the tunnel would cost about $1.5 billion and the bridge $750 million. In either event, however, both junctions would have to be squeezed between current infrastructure.
It is also my estimation that it is infeasible to attempt to bring 30th St. Station's north throat to speeds greater than about 120 mph (which would still lose the 1.5 minutes currently expended traversing Zoo). Both the bridge and tunnel routings I offer are likely going to have to be that speed.