Positive Train Control and last week's Hoboken crash

Last Thursday's wreck in Hoboken, NJ, where a New Jersey Transit (NJT) train overshot the end of track and crashed into the platform and station building has renewed mainstream conversation — and confusion — about Positive Train Control.

First, we don’t know the cause of this crash and this article will not speculate. Here’s what we do know:

• On September 29th, NJT train No. 1614, a diesel-hauled push-pull set, was operating in push mode, with the engineer operating the diesel locomotive from the cab car at the front of the train.

• At about 8:45 a.m., the train failed to stop at the end of the track in Hoboken, NJ, and continued onto the platform, knocking down part of the canopy. A bystander was killed by flying debris.

• Cab signals and Automatic Train Control are active in the Hoboken terminal. A Positive Train Control system is not.

Continuous cab signals were developed by United Switch & Signal and the Pennsylvania Railroad beginning in 1922. The system uses the on-off pulsing of a low-voltage circuit in the rails to continuously send a signal indication to a train. They provide the safety and efficiency benefits of a constant reminder of the signal the train is operating under, and the ability to upgrade and allow the train to speed up if the next signal improves.

A modern Aspect Display Unit on an MBTA cab car equipped with continuous cab signals, Automatic Train Stop, and ACSES. Together these three systems comprise a form of Positive Train Control. The top number (30) is the allowed speed and the middle number is the train's current speed (0).

In modern practice, the signal indication conveyed is virtually synonymous with allowable speed. While older in-cab displays are representations of signals, most new displays juxtapose the allowable speed alongside the speedometer (pictured). Speeds range from 150 mph to 20 mph. There is no code for stop.

Automatic Train Control (ATC) is an enforcement overlay on top of continuous cab signals. It ensures that trains are not operated in excess of the allowed signal speed displayed in the cab. When the cab signal downgrades, the engineer must acknowledge the decreased limit. If the train is moving over the new speed limit, the engineer must reduce speed. If he or she fails to do so, the system will make a penalty brake application which also trips the Pneumatic Control Switch (PCS), shutting off the locomotive’s propulsion power. The engineer can suppress this penalty application and recover power but without this action the train will stop.

Because there is no cab signal code for stop, ATC cannot be used to force a train to stop. As long as the train is kept below the 20 mph limit imposed by a Restricting cab signal, it can pass stop signals or the end of track or collide with other movements. Cab signals and ATC are used for speed control, not collision avoidance or stop signal enforcement.

Positive Train Control (PTC) refers to any system or combination of systems that provides comprehensive enforcement of signal speed, civil speed (including permanent speed restrictions such as curves and temporary speed restrictions such as track work areas), and positive stops. The 2008 Chatsworth, CA, collision prompted a federal mandate of Positive Train Control nationwide by mid-December 2015. In October 2015 with the deadline nearing and railroads nowhere close to having PTC developed, let alone installed, the deadline was extended to December 31, 2018.

The predominant PTC system among northeastern passenger railroads is Amtrak’s Advanced Civil Speed Enforcement System, or ACSES (pronounced “access”). Originally developed by PHW and Alstom for Acela Express service, it is equipped on all Amtrak-owned trackage on the Northeast Corridor and most commuter railroads that use its track. New Jersey Transit is installing an interoperable but slightly different system known as the Advanced Speed Enforcement System (ASES) which is not active in Hoboken at this time.

ASES is still in its development and installation phase with little technical information publically available, so this article will discuss the functionally similar and better-known ACSES assuming, except where mentioned otherwise, that ASES works on the same principles.

Both ASES and ACSES are a third overlay on continuous cab signals and ATC, providing civil speed and positive stop enforcement. Civil speed is beyond the scope of this article, but here’s what you need to know about positive stop enforcement.

• Every train so equipped knows its exact location on the railroad. Wheel rotations are tracked by a computer on a detailed route map and any inaccuracies from wheelslip, wheel wear, or track corrected by periodic track-mounted transponders (a “balise”) which serve as landmarks.

• Being aware of location, a train knows when it’s approaching a signal (including a permanent stop at the end of track). When its cab signal system is receiving a Restricting code (20 mph) and ACSES knows the train is approaching a signal, the system puts two and two together to determine that a stop is required (on ACSES, the train will also receive a data radio signal confirming a stop).

• Approaching decreased speed limits or a location where a stop is required, ACSES ensures that trains slow down or stop in time by calculating and enforcing a braking curve — a vast improvement over the choppy downgrades imposed by simple ATC. The engineer’s display shows a real-time speed limit based on how far the train is from the point where a speed or stop is enforced. If the train is operated within the curve, the system takes no action, but if the speed exceeds the braking curve, a penalty application will be made.

• A Positive Train Stop Zone precedes the stop signal or end of track, where the ACSES braking curve hits zero and all trains are forcibly stopped with a penalty application.