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Sunday, June 8, 2014

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.


  1. Why do you call it lightweight? Between the code generators, the transmitter/receivers at the bonds, and the carborne equipment it's a fairly infrastructure dependent system. Track mounted balises communicated to a dispatching computer via radio involves far less infrastructure.

    Or are you referring to it being lightweight in terms of the rule book? Cause its not that either. It's just cutting out a safety device and putting the burden of maintaining train separation on the dispatcher through their sparse home signals.

    1. I'm referring to the procedures which don't require train order forms, a pilotman or even a 'C' board.