I don’t know much about Plane 8 aside from what I see in these images from May of 1970. They were all labeled “Foundations at Plane 8.” The last image is labeled, “bridge at foot of Plane 8.”
From a site called Wikimapia it shows that the bottom of the plane was at what was known as Muleshoe Curve. The Muleshoe was a relative of the more famous Horseshoe Curve, built in the 1850s and closed in 1981. The Muleshoe and the alignment of the portage railroad can be seen on this 1902 map of the Ebensburg quadrant topo map.
The National Park Service has a lengthy report on-line about the APRR. You can download it here. The report is from 1973 so I wonder if these excavations were part of the study.
If any of this remains intact, let me know. There is a trail along the old railroad in this area and I am surprised not to find more on-line concerning it.