well, here goes, heading south on 81 from the 380 interchange in Scranton: I am going by maps, so I am not sure what you can actually see from the road.
About a mile south of 380, the old Delaware Lackawanna and Western (DLW) mainline, is first on your left and then passes under, on its way to Scranton. Operated by shortline "Delaware Lackawanna", and also used for occasional excursions by the Steamtown museum in Scranton
Between exists 182 and 185, crossing under from R (coming from Scranton where it connects to the DLW mainline) to L is the old "Laurel Line" from Scranton to Wilkes Barre, now operated by Delaware Lackawanna. Just past 182 the line crosses back under again from R to L, to serve a few industries there. That's rest of the line to W.B. is gone I think.
Just south of exit 180, a branch of the old Erie RR crosses under. Now served by Luzerene Susquhehanna (LS) for freight, a local shortline contracted by county governments to run old lines in those counties
South of 175, the old Lehigh Valley RR "Cutoff" passes under, used to be part of their freight mainline from NJ to Allentown up to Buffalo. Now part of the mainline of Reading and Northern. Perhaps 6 trains a day or more. Most active in this I think.
In the Ashley area (164) the mainlines of the Central RR of NJ and Lehigh Valley (passenger main, which was split from the freight main in the W.B. area) crossed under. They are both long gone, not sure anything is left to see. There is also a tunnel under I-81 somewhere in the area of a streetcar line from Wilkes Barre to Hazelton that also handled freight - part of the tunnel still exists and you see posts on the web about it now and then.
A miles south of Exit 145, in the Hazelton area, a branch of the LV crosses under, now operated by Reading and Northern. Just south of there, west of Hazelton, the R&N tracks run close to I81 on the east (left) side for maybe 1/2 mile. Again, not sure what you can see
A mile south of Exit 143, another branch of LVRR (now R&N) passes under going from Hazleton to an industrial park. Very active, trains almost every day.
a mile south, exit 138, another LV branch (now R&N) from Hazelton passes under, then runs next to the highway on the NW side for a mile or two, then passes under again. (Also near here there is an abandoned tunnel under I-81 and the rest of the top of mountain, of an abandoned Reading RR branch.) Just before exit 134, this LV branch passes under yet again . It then skirts the right (NW) side of the highway a mile or so , and it eventually ends
not much farther at a coal mine south of Mahanoy City .
At Frackville (124), branches of the Reading RR and the Pennsylvania RR used to cross South to North. Nothing is operated there anymore - I am not sure if anything was still operated when I-81 was built, and there were bridges for the RR, or if everything was obliterated by I81.
A mile north of Exit 107, a R&N line for coal crosses under (used to be part of Reading RR)
In the milepost 106-104 stretch, I think there used to be Reading RR branches through the valley, but may have been long gone before I81. Also a few others like this down to the I-78 junction.
Well, that's enough for now. Someone please correct all the errors and omissions I made, and fill in the rest of the way.