CR 60th St. I.T., formerly parts of the PRR 60th St Branch and Chester & Philadelphia Branch. Both were built in 1917-18, to serve wartime industry along the Delaware, especially the (boondoggle) shipyard on Hog Island, but were relocated on a regular basis when the airport was built and expanded.
What's left is all (relocated) Chester & Philadelphia Branch. Around 1947, the 60th St Branch was relocated to run parallel to Island Avenue and tie into the Chester & Philadelphia Branch at a wye just below Fort Mifflin. Some of the track in that vicinity survived into the Conrail era as the "Enterprise Industrial Lead" serving the sewage treatment plant and warehouses on the east side of the airport, but that looks to have been severed by expansion in the late 1990s.
In addition to the Sun Oil tank farm at the north end, there's a spur curving off onto Hog Island to handle Corps of Engineers dredge spoil, which was being shipped for strip mine fill or something like that. There used to be a spur closer to Fort Mifflin proper, legacy I think of WWII when there was a loop there for munitions loading, but that also disappeared in the late 1990s.