Caught on Camera: Track Circuits and Electrification Current

In a previous post covering the phenomena of a "bobbing" track signal that cycles intermittently between one or more states. I mostly covered the spectacle of a particularly egregious example on SEPTA's former Reading Main Line. Here I will cover two additional examples that are a bit more tame, but sheds some light into some operational side effects that come with railroad electrification. 

PATCO FERRY interlocking with an eastbound train on the troublesone WF2 track circuit.

The first example was caught on the PATCO Speedline and involves the cab signal bobbing between 30mph (Approach) to 65 mph (Clear) immediately after a westbound train crossed over to the eastbound track just past the Ferry Avenue interlocking. Was this cased by standing water or a lose wire? Apparently this bobbing signal is a known issue that occurs when the operator or ATO system selects the P4 power setting after exiting the turnout.  The current draw causes the WF2 track circuit relay at West Ferry interlocking to drop.  With the WF2 and NJ67 track circuits both occupied, the signaling system thinks that the train is still on the 30mph switch at West Ferry and drops the cab signal from Clear to Approach.  The train cuts power, the WF2 relay picks up, the signaling system "sees" the train off the 30mph switch and upgrades the cab signal to Clear again.  The process continues until the train passes the next code change point.

As I explained in my post regarding impedance bonds, the rails serve double duty as both a path for traction return current and the track circuit. Because of this a mis-calibrated signaling component can be affected by the acceleration state of nearby trains.    A "safe" failure of this type is less likely to be tested for upon installation or fixed quickly once identifies.  A good example of this is the southern segment of Amtrak's Northeast Corridor that still utilizes relay--based signaling hardware from the early 1980's.

Like with the PATCO example I have been told that the interaction between the signaling system and electrification can be to blame, especially from trains traveling on adjacent tracks. The first example was located at the northbound Milepost 69 intermediate signal near Aberdeen on the less frequently utilized center track #2. The signal can be seen slowly cycling between Clear and Approach, potentially due to an electric train movement on one of the outer "through "tracks". 

Here we see a slightly more extreme example as the southbound Milepost 103 intermediate signal on track #2 at Halethorpe rapidly flickers between Approach and Clear. It might look like Advance Approach is trying to display, but the signal blocks ahead are of usual length and the flicker interval is not uniform. This was noticed after the passage of a southbound Amtrak electric train on track #1. This instance of signal bobbing would also be likely to avoid rapid resolution as track #2 is less frequently used than tracks 1 and 3 and for at least half the day traffic is set in the opposite direction. 

Milepost 103 signal location with track #2 set northbound

Years ago when I had access to Amtrak NEC reports, there would be bobbing signal issues that would persist indefinitely. When the cause is an intermediate side effect of the electric traction system, these problems can become very difficult to troubleshoot as they might require a train running at some specific power load/speed to trigger the problem. A big reason central European countries are so enamoured with axle counters is that many electrified before installing automatic signaling. Counters solve both the problems associated with impedance bonds, but also problems like bobbing signals.