As has been pretty clearly described, this was a connecting track between the PRR's Port Road and the ex-B&O Philadelphia line. It was used erratically for detours and, in fact, was originally built for that purpose in 1908. At that time the B&O was rebuilding its Susquehanna River bridge, completely replacing the old single-track iron spans with the steel double-track structure that exists today. But on 9/23/08 one of the spans collapsed under a coal train, putting the bridge out of service and requiring an alternate route.
On the south (west) end, a connecting track to the PRR was built at Swan Creek, and at the east end, the steeply graded line at Perryville. Helper engines were stationed at Perryville, but as you might imagine, it was a slow, laborious detour. I'm not sure how long this original detour lasted, but it may have been as long as a year or so. It's possible that after the collapse, the engineers decided not to try to replace the bridge under traffic. Whatever, the new bridge opened in January 1910.
Afterward the Aiken/Perryville connection was retained and occasionally used, but I have no info on how often or under what circumstances. The Swan Creek connection was removed earlier, and as far as I know there's little or no trace of it now.