Rail systems, whether freight or passenger rail, are complex systems, requiring the management of the movement of thousands of pieces of equipment over many miles of rail. Signal systems assist with coordinating these movements by providing instructions to vehicle engineers and feedback to dispatchers about vehicle movement.
However, as human beings are inherently “human”, mistakes can be made, leading to accidents. Signal systems are also not perfect, and although systems are designed to provide false positives in the event of failures, system failures are still possible.
What is Positive Train Control?
Positive train control (PTC) is a term that refers to an assortment of technologies that result in an automated, complete stop of the train if it will enter a condition that could lead to an accident. These conditions include the engineer not heeding a signal, a switch not being fully engaged, or other conditions of vehicle or track. In theory, PTC systems will detect the presence of a negative condition and depending on the severity of the situation, alert the engineer or attempt an emergency stop of the train.
PTC systems generally consist of equipment on the rail vehicle, equipment in the control center, equipment on the rail wayside, and wireless communication between all three elements. Global positioning systems (GPS) provide location information for each vehicle to the control center. The PTC system essentially reviews speeds, track conditions, and vehicle locations and determines braking curves for each vehicle. The system alerts the vehicle crew if a condition presents itself that requires the train to slow or stop. If the crew does not respond, emergency braking is applied.
Variations in Positive Train Control Requirements
Depending on the type of railroad, the functionality and requirements of a PTC system can vary significantly. Freight railroads generally perform fewer switching operations and have fewer stops, but due to the extremely large loads they carry, are sensitive to stopping distances, which can often be measured in miles. Passenger railroads are more concerned about maintaining adequate distances between trains while maintaining acceptable operation schedules. Passenger railroads also have the added issue of liability and costs associated with injuries and deaths associated with accidents.
Issues with Positive Train Control
Implementing PTC systems across a railroad can be an expensive proposition, particularly for smaller rail systems that do not have large operating budgets. The technology has to be integrated into vehicles and control systems, potentially requiring upgrades in auxiliary equipment and software.
PTC systems are also not perfect. The June 2009 collision of two Washington Metro trains resulted in at least 9 deaths and many injuries. Initial investigations show that the striking train was under automatic control, and there is evidence that the emergency breaking system was applied prior to the collision. It is unknown at this time what the true cause of the collision was, but since PTC systems are designed to avoid these types of accidents, it can be surmised that there was a failure somewhere in the system.
Mandates for Positive Train Control
Currently, PTC systems are being implemented throughout the United States on a purely voluntary basis. The types and degrees of PTC systems vary widely depending on the type of railroad, rolling stock, conditions, and other requirements. However, the Rail Safety Improvement Act of 2008 (RSIA) mandates the installation of PTC systems by the end of 2015. The Federal Railroad Administration (FRA) oversees the development of PTC systems by US railroads, and has funding available to subsidize the implementation of a potentially costly technology.
The proper design and implementation of PTC systems can prevent accidents from occurring in passenger and freight rail systems. The initial capital expenditure in these systems are insurance protecting the railroad against costly accidents.
Sources
Federal Railroad Administration PTC website
Ten 90 Solutions website
Susan Kristoff - Susan Kristoff is mechanical engineer by trade, but has a diverse set of professional and personal interests. The glue that binds all of ...
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