NS (N&W/NKP) abandoned theirs when all they needed to cross the river for, essentially, was Buffalo Junction Yard. At the time, it made sense to strike a deal with Conrail to use the BCK bridge. Also at the time, as I mentioned above, the south leg of the Buffalo Line to Seneca Yard made sense for Conrail because it was all theirs, so they could logically use Seneca to assemble trains headed down that line and take the southern belt. The overpass was configured in such a way that this made sense to them. They could also use the southern leg and the Ebenezer Secondary or the Compromise if they needed to bypass CP Draw. When NS took the Buffalo and Southern Tier lines they kind of got put up the creek without a paddle because it meant more traffic over the bridge to Bison yard and the Buffalo line. The lines that would have remotely allowed them access to the southern "belt" of the Buffalo Line had long been removed, namely the PRR line that would've connected just north of the current Buffalo Line bridge over NS, was already long removed and honestly connected in a way that wouldn't have been useful anyway. Even with these original connections in place (the LV and PRR connections to the southern belt in Lackawanna) they wouldn't have made any sense for NS to be able to use, logistically. They were still better off using the Bison and Ebenezer tracks they do today.
On the second part, there's no way that would ever make sense for today's layout either. If the LV was intact from Lackawanna north, there'd crossover for CSX and NS where NS wanted to get from the LV/PRR half to the NYC portion northward, and CSX wanted to get from the NYC half to the LV line northward. Even beyond that, CSX using the LV connection north would drop them at either the old Erie or DL&W yards at Bison, doing them absolutely no good. CSX using the NYC connection instead through the old Gardenville Yards would drop them at the main line on the wrong side and facing the wrong direction of Frontier yard.
While we're slightly off topic of the Gardenville line and sort of on connections to the west over the creek, here's a pipe dream for you. What really needs to happen is CSX and NS need to sit down and hammer out a deal whereby they
- Jointly build a new bridge in place of the NKP bridge and configure some crossovers that allow CSX and NS to use the northern bridge.
- Since NS owns the Buffalo Line still, they need to help B&P with constructing a "Martin Road Connection" to get them from the Buffalo line to the old B&O main, in preparation for the next step.
- Take the CP Draw bridge out and build a new one in its place to accommodate NS, B&P, BSOR & SBRR (if the latter two even use it anymore). Since NS and B&P would already have their connections from the above two steps, this would only be a temporary bottleneck over the first new bridge until the second new bridge is opened.
- Once the second bridge is opened, CSX would take primary ownership of the north bridge while NS takes primary ownership of the south bridge, but connections and agreements remain that CSX could use the south bridge if needed and NS could use the north bridge if needed. B&P, SBRR, and BSOR could continue to use the south bridge if they wanted.
The only reason I preclude B&P/SB/BS from using the north bridge is that NS has less traffic so would probably be a better fit for trackage rights. Also with the exception of BSOR, I don't think B&P and SBRR would logically be able to get new connections to a bridge in that northern spot without some really tight and convoluted crossovers.
But I digress. The unfortunate answers overall are:
- without using the old Gardenville yards, the old configuration of the Gardenville line served no purpose to NYC, Conrail OR CSX as a connector between Lackawanna and Depew and likely never would.
- Without a connection to the NS line or any of the yards, the new configuration of the PRR Buffalo Line serves no purpose to NS or B&P as a connector between Lackawanna and GJ.
- In order for this line to ever be viable to B&P, a connection needs to be built at Martin Road to tie the PRR Buffalo Line to the B&O Third Main to access Buffalo Creek Yard.
- If some absolutely horrible catastrophe were to befall CP-DRAW, NS could in a pinch use the Martin Road connector and Buffalo Creek Yard to run trains on a long roundabout path from Bison yard over the Transco Wye, down the Ebenezer, to the Buffalo Line, up the B&O, then turn the trains in Buffalo Creek Yard and head south down the Lake Erie District, but this would be extremely complex in operations given limitations at the Transco and sharing the BCY with B&P.
- IF (that's a BIG IF) NS ever wanted to permenantly bypass CP-DRAW and head straight to Bison Yard, they could potentially rebuilt the PRR/LV line from Lackawanna to William St and come into Bison from that side, but again, they'd have to turn all trains there if they didn't use the Bison Runner over CP Draw.