PTC Capacity Impact Visualized

 I've frequently pointed out the impacts of Positive Train Control on train performance in general. While not technically inherent to the requirement, they are an inevitable consequence of policy and implementation choices. These include things such as stacked safety margins, overly conservative performance assumptions and locations tracking uncertainty. While the ACSES system used by cab signal equipped railroads in the northeast is generally superior to the wireless data based ETMS used by the class 1 freight railroads, certain ACSES implementations have continued to demonstrate the performance problems I identified almost a decade ago. Recently I was able to capture a concrete example of one on video.

Here we see an NJT Morris and Essex train led by ALD-45 #4510 entering Newark Broad St station on Track #2. The 4E signal for BROAD interlocking is immediately east of the platform end and the eastbound train gets hit by a positive stop about 700 feet (6-8) carlengths short of the signal location. Instead of being able to complete its station work as the signal system was designed to allow, the train sits off the platform for over 2 minutes, adding to the delay.

ACSES implements its positive stop feature using a two step process.  First a fixed track mounted beacon that transmits other permanent speed and positioning data will inform the onboard system that the train is approaching a positive stop. Next, if no cab signal code is detected , the onboard system will enforce a positive stop by calculating a 0mph stop point based on its super conservative assumptions of ice covered rails and a train with cheese in place of brake pads. The train stops up to 1500 feet from the signal and, well, the current policy is to wait there. The initial concept was for crews to use a stop release procedure to creep up on the signal, a feature implemented by specific freight railroads using ETMS, however most most if not all northeast commuter railroads have taken the CYA approach and just let the trains sit several hundred feet in advance of the signal, even if that means being unable to platform.

Supplementary ACSES transponders at Valley Stream

Apart from stop release, one alternative mitigation is to add additional ACSES transponders that can reduce the location uncertainty. The LIRR has installed two additional sets at Valley Stream that sees the same problem with platform-end signals, although I have heard this can still stop trains 1 or 2 carengths short of the stop point. A quick look at the overhead shows that NJT has not implemented this mitigation at Newark Broad Street. Another mitigation is to set the stop point to the legal requirement of the fouling point of the first trailing point switch or diamond. At NJT's BROAD interlocking this point is about 1 carlength beyond the 4E signal. Finally, they can be more realistic about the performance of the equipment. 

Just like the New York City Subway a decade ago, its always safer to cover ones ass than to fight for performance in a post-pandemic transit landscape were trains are running half empty anyway. If NJT feels like it is looking at service cuts, investing in capacity makes no sense.