PTC: overlay, or stand-alone?

Railway Age, May, 2004 by Roger W. Baugher

In his well-considered Point of View (RA, June 2003, p. 76), Amtrak's Larry Light traced signal system development from early block circuits in 1872 through implementation of trustworthy signals after 1907. In this period, signals transformed from informational devices to fail-safe (vital) devices, where lives depended on their accuracy and reliability. Cleverly, the system's vital functions were implemented in the field, close to the events and devices--train movements, signals, turnouts, etc.--with which they interacted. Communication with central offices permitted dispatchers to control actions in the field, but the communication link was not vital--it was necessary for efficient operation but not for salt operation.

Technology now enables us to devise new control mechanisms such as Positive Train Control (PTC). In these systems, computers enforce authority limits and speed restrictions, bringing a train to a stop or a lower speed if the engineer fails to act appropriately. The technology can even permit retirement of wayside, fixed-block systems and replacement with moving-block systems where trains moving below maximum track speed follow more closely since stopping distances are shorter. Light reminds us that such technologies require that the system's vital functions be moved to computer and communication platforms, away from the current wayside system vitality that has served us reliably, safely, and efficiently to date.

Are the savings from wayside signal elimination and the benefits of moving-block sufficient to justify the high cost of new, fall-safe systems? Or can many of the benefits of PTC be achieved without abandoning the current vital platforms?

Analysis of CSX's Communications-Based Train Management (CBTM) system shows that overlay systems provide a solid economic return while avoiding the cost of instituting new vital systems. The elegance of this approach is that the failure of any component of CBTM simply requires the railroad to fall back to its current operational methods and rules. The railroad continues to operate, but less efficiently. In contrast, the failure of any component of a vital PTC system not only jeopardizes safety, but requires the railroad to stop completely until the system is restored.

Consider a highway example--a traffic signal. It is one component of a set of rules that specifies how we are to operate our automobiles. When the signal functions correctly, traffic moves far more efficiently than if the signal was not present. However, if it fails, drivers know to treat the signal like a four-way stop sign. Traffic continues to move, albeit less efficiently, but safety is not jeopardized. Here, the traffic signal is a non-vital device, effectively an overlay to existing rules of highway-traffic operation. The vitality of the system lies with each car's driver.

The vital version of traffic signals is an Intelligent Highway System, where the car, not the driver, is told what speed to maintain at a location. Because the speed is set by the system, vehicles follow at a distance commensurate with the speed, much more closely than in a non vital environment. The greater efficiency comes at a high price. Besides the costly deployment, failure of any component, like a flat tire on a single automobile, jeopardizes safety and literally brings the system to a crashing halt. There is no fallback, simply failure.

Applying this analogy to railroading, CBTM provides most of the benefits of a full-blown vital PTC system, but at a lower deployment and maintenance cost. Ultimately, its inability to provide moving-block or permit the removal of wayside signals may be unimportant. Few corridors have traffic density or a freight/passenger mix that warrant moving block. If needed, this capability can be provided efficiently with existing signal systems having shorter block lengths and advance aspects, supplemented with other overlay systems, such as the cab signal-based ACSES in the Northeast Corridor. While retirement of existing wayside signals may be desirable, their retention provides the broken rail protection not otherwise available in PTC systems.

Clearly, CBTM is a harbinger of things to come, with CSX deploying it on both signaled and unsignaled territory, enroute to its ultimate goal of system wide implementation. Western roads are interested as well, with BNSF implementing ETMS, a similar system, on a high-density freight corridor. It is poised to become the PTC standard for freight railroads in the U.S., and a key stepping stone to more advanced PTC implementations where the higher cost of such systems can be justified by the mix of freight and passenger traffic.

Roger W. Baugher, president of RWB Consulting, Inc., has 28 years experience with signal systems and dispatching issues at three railroads and in consulting. At CSX, he performed an economic justification of CBTM.