Pittsburgh Line Cab Signal Overlaps and Human Factors

A recent derailment between three NS freight trains near Labrobe, PA sent me down the rabbit hole of human factors and accident prevention once again.  The derailment was the result of a restricted speed rear end collision that caused cars to derail into the path of a third opposing freight train.  Straight away I would like to point out that this sort of accident is not PTC preventable and exactly the reason  why high crash standards are important even in the face of technical safety systems.

Occurring just a few months after NS changed the Pittsburgh Line from Rule 261 wayside signaling with Cab Signals to Cab Signals without fixed automatic wayside signals I immediately suspected that the loss of visual cues from wayside signals prompted a crew to misjudge the point where Restricted speed would take effect as under the traditional wayside system an obstruction can be located one foot beyond the signal displaying Restricted Proceed.

 In theory a cab signal should change from Approach to Restricting as one crosses the insulated rail joint into the block where the cab signal code is not reaching the following locomotive.  It is standard practice for a cab signal cut to be placed some distance in advance of an absolute signal, dropping the cab to restricting in time for a train Approaching at Approach to know that an absolute stop is necessary.  I have observed SEPTA placing cab signal cuts in advance of automatic signals on the Reading viaduct, but there the signals are spaced a half mile apart so and SEPTA had plans to convert the Regional Rail Division to transit style operation anyway.  However, in doing some research I learned that Conrail had implemented this same system on its entire Pittsburgh Line with cab signal cuts between every signaling location such that a train on Approach would see its cabs drop to Restricting a mile before encountering the Stop and Proceed signal.  This feature has since been confirmed from a head end video.

New Signal Indication Point (SIP) at MP 125.3

Old unlabeled Cab Signal change point at MP 125.3

This explains why NS was creating new "Signal Indication Points" every mile along the Pittsburgh Line as part of its re-signaling project.  It wasn't increasing capacity by increasing the number of blocks, just replicating the system Conrail had already installed.  I have long wondered how Conrail would prevent rear-end accidents on its Rule 562 territory (cab signals without fixed wayside signals) as there were no mentioned in the rulebook of crews traveling on Approach being provided with a stop target (as is the case on the Union Pacific Hiawatha Sub).  It turns out that they were likely using signal overlaps the whole time.  If this practice originated with the PRR's Conemaugh Line experiment remains to be seen.

Of course when it comes to human factor's nothing is what it seems.  In a world where one's cabs stop to Restricting halfway through a block one knows to be clear until the next wayside signal, crews might not take that Restricting cab signal so seriously until passing a wayside Stop and Proceed.  Over decades of service, these habits can become ingrained, especially if rules testing fails to cover the corner case.  A feature designed to provide crews with advance warning of a track obstruction may have instead created a false sense of security and a hidden reliance on wayside signals.

Additionally, although this episode both revealed and in part confirmed the use of cab signal overlaps on former Conrail territory, it now raises further questions about how the new NS SIP system will function compared to the Conrail system of waysides and cuts.  Will the "odd" SIPs still only act as Approach -> Restricting cut points or will they be sully featured automatic signal locations that help to reduce train separation?  I guess I'll have to put my feelers out ;-)