Caught on Camera: Bobbing SEPTA Main Line Signals

Several years ago I discussed the topic of relay logic and how it can create interesting signal displays as they change from one aspect to the next.  (Usually this involves a change from some form of Clear to some form of Restricted Proceed as that involves two or more discrete relay flips.)  Well last week I had taken a trip to the SEPTA North Broad station, just south of the busy 16TH ST JCT, to photograph the last remaining AEM-7 locomotives in operation.  At one point, towards the tail end of the peak period, the track 1 signal on the Milepost 2.9 automatic signal bridge began to cycle between Approach Medium, Approach and Stop and Proceed.  The northbound home signal at 16TH ST was displaying Medium Clear, so Approach Medium was indeed the correct indication, however the signal continued to move between the three at a fairly brisk clip indicating that the track circuit between there and the interlocking was moving between an occupied and an unoccupied state, a phenomena known as "bobbing".



As time went on the rate of the cycling increased and as soon as the AEM-7 led push-pull on the adjacent track 2 cleared tthe approach block to 16TH ST, that signal began to bob as well, although only between Approach and Stop and Proceed. In due time a northbound train approached the 29-1 signal and I can only imagine what the crew was thinking as they not only watched the wayside signal change ahead of them, but also endured a constant stream of cab signal flips. As one might have expected, the train passed the malfunctioning signal at Restricted speed and shortly thereafter the track 2 signal was also brought down to the Stop and Proceed position full time by an adjacent northbound train.



After the two trains passed whatever temporary fault condition that existed was resolved and the MP 2.9 automatic signals went back to normal operation.  There was a later service disruption at the junction, but it appeared to be related to some sort of stuck switch or disabled train. The funny thing was that this wasn't even my only recent encounter with bobbing signals as I also caught two northbound signals at Milepost 69.6 on the Amtrak's Southern NEC also bobbing.



Some bobbing track circuits can be fixed with a few simple adjustments.  Others can be quite stubborn and can linger for weeks.  Some parts of the southern NEC had bobbing circuit conditions that had been around for years, often where electric movements on one trackcould cause an adjacent track to temporarily show occupied.