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. 

PATCO CENTER Tower Moves To The Pherphery

 As the prototype for the "modern" heavy rail rapid transit system, Philadelphia's PATCO (Port Authority Transit Corporation) Hi-Speed Line was naturally designed with state of the art signaling. However as a prototype it had a few features rooted firmly in the past. For example it had cab signals without wayside block signals, but it used PRR style 100hz pulse codes as opposed to the audio-frequency track circuits that would appear on the DC Metro and BART. One of PATCO's most visible links to traditional railroad operation was its signaling control where the full-line CTC was placed not in a command center or dispatch office, but a "tower" built and named for the PRR interlocking tower that had occupied the same location for the previous 40+ years. 

As built by the Pennsylvania Railroad, CENTER tower sat at the 3-way junction where trains from the Federal Street Terminal would fan out en-route to various points along the South Jersey shore from Long Beach Island to Cape May. Older than the wave of towers that accompanied with 1934 PRSL merger, CENTER featured a 66-lever EP machine and despite the drop in passenger traffic throughout the 1950's, still saw plenty of action from freight trains and lite engine moves to the PRSL's primary service facility at the old Federal Street complex.  

This would change in 1966 when construction on the PATCO Hi-Speed line commenced. The project would replace the PRSL's Broadway station and the associated trackwork controlled by the old CENTER. The resulting real estate was repurposed for Interstate 676 and all traces of the old CENTER tower was erased, however the spirit of CENTER would live on in the new PATCO Hi Speed Line. 

When completed, the PATCO Speedline consisted of the existing 1936 vintage Delaware River Joint Commission "Bridge Line" between Camden and downtown Philadelphia along with a brand new grade separated rapid transit line to the New Jersey suburb of Lindenwold.  The new and old parts of the line joined just east of the old PRSL Broadway station in a space that would have been physically within the territory of CENTER tower. As PATCO tried to re-use as much existing infrastructure as possible it was decided to build the CTC office at this junction point as it could directly interface with the existing signaling and telecom links from the Bridge Line and not have to somehow forward those signals all the way to Lindenwold. Because PATCO's design team were generally rail enthusiasts, they named their CTC office after the old PRR CENTER tower. 

Technically the structure did count as a traditional tower as the operating floor was located above a typical PATCO substation. In addition to the unit lever CTC machine, CENTER also supported traction power direction and remote customer service via "call for aid" telephones and CCTV. CENTER would serve PATCO well for over 50 years, however as the systems were upgraded over the decades, the rationale for its existence, supporting direct wire control cabling, would no longer apply. Instead CENTER would be physically isolated from the main PATCO offices and shoppes in Lindenwold and also located in what had become one of the most dangerous cities in America. With the building starting to age and the work-site unpopular with employees, PATCO eventually found the money to build a brand new state of the art control facility at Lindenwold.   

Although the original CENTER tower will be retained as a backup facility, much of the spirit of classic railroading that CENTER represented is likely to be lost.

Lightweight Cab Signaling Resiliency

The more we seek to use "high tech" signaling methods like coded track circuits or communications based train control, the more we run the risk of creating fragile systems that can't handle failures well.  When a signaling system is not resilient it means that service must be canceled and substituted with buses, if at all.  This is commonplace in certain parts of Europe like the UK where overly restrictive rules require tokens and pilotmen to work against the flow of traffic or where signaling rules have been suspended.

The Port Authority Transit Corporation or PATCO operating across the Delaware River from Philadelphia was one the world's most advanced transit system in 1968 pioneering such innovations as one person operation, automated fare control and collection, automatic train operation and, of course, automatic cab signaling without fixed waysides (except at interlockings).  While based on the proven pulse code cab signaling system adopted by the Pennsylvania Railroad, cab signals can fail from time to time and this requires some sort of contingency operation.  On sections of the PRR that attempted the same type of operation they employed the  now well known 'C' lamp (C for Clear to Next Interlocking), which could be displayed by the dispatcher in case of a failure of the carborne equipment.

However there are limitations to the 'C' lamp as in it won't display if there is a track occupancy light in the line segment so without that trains typically need some sort of track warrant plus another dispatcher interaction to get permission past the stop signal.

So anyway I have always admired PATCO for being one of the most flexible and resilient rail transportation systems around.   Once when a train was stuck on the river crossing they used one from the opposite headway to head out and rescue it.  Once when there was a switch problem my train load of people literally swapped places with another train across the platform and then continued on towards our destination with cab signals and automatic train control cut out.  PATCO once didn't raise fares for 15 years, got 35 years out of its first set of ties and has 46 years on its original rolling stock. 

About a month ago on National Train Day PATCO was again hit with a minor operating problem, but in typical PATCO style they soldiered on like it was no big deal.  A signaling problem on the Ben Franklin Bridge rendered rendered it impossible for the dispatcher to reverse "traffic" on the bridge so eastbound trains could neither get a signal at the 8th and Market interlocking, but would also be stopped by cab signals the whole way across.  To deal with this problem the dispatcher simply authorized the operator to cut out all the safety system and operate past a stop signal and then without signal protection across the bridge.  Luckily I was filming as the operation transpired. 



At 00:53 the train gets a diverging proceed signal to cross over at the 12th St interlocking.  At 03:40 the train gets stopped by the PTC system to stop the train in the overlap before the stop signal, but a simple verbal instruction from the dispatcher and the operator can cut out the safety system and platform properly.  At 05:00 the operator get permission to operate under Home Signal Block rules (aka clear to next interlocking) and past the 8R signal at Stop.  After a quick readback we are on our way at 05:40.  We then operate at normal speed until 10:20 where normal operation resumed.



In this followup video there is a second segment of single tracking for an unrelated issue and at 00:50 we are stopped again by the PTC system, but before we even come to a stop the dispatcher is on the horn authorizing another bypass.  With basic procedures the service is not disrupted, costs are saved, passengers are happy and nothing collided, derailed or exploded.  This represents the gold standard for flexibility and resiliency in the face of a cab signaling failure.