Rail transit systems heavily rely on Automatic Train Control (ATC) systems to ensure safe and efficient train operations. A critical component of the ATC system is the Track Circuit, responsible for train detection and speed control. However, challenges such as Track Circuit Bobbing and Loss of Shunt compromise system reliability and impact revenue service. This paper addresses some of these challenges and proposes solutions based on real-world data analysis.


Track Circuit Bobbing: Short-term occupancy fluctuations when the track circuit is unoccupied. It can be caused by aging equipment, dissimilar rail, bond saturation, improper track circuit adjustment, and uneven Traction power return in the interlocking or through the wayside track circuit impedance bonds. Trains lose or reduce speed command due to bobbing track circuit, affecting revenue service. Current Imbalance causing the current flow in the impedance bonds causes impedance bond saturation. The impedance bond saturation begins at 100A of imbalance; 300A can lead to track circuit temporary failure. The resolution can be by analyzing the Transmitted and received signals and by performing thorough Visual inspection of the wayside and TCR components along with replacement of impedance bonds, other components, and rail as needed.

Loss of Shunt: The failure of Track Circuit to detect trains, leading to false clear indications. The causes could be Corrugated rail, arching third rail, improper track circuit adjustment, broken wayside components, and defective ATP components, difference in the Running rail elevation or uneven rail to wheel connection. In WMATA the LOS is monitored using LOS monitoring tool developed by Metro’s signals engineers Johann Glansdorp and Tim Shoppa.


Track Circuit Monitoring Tool - By using data techniques to tackle Track Circuit Bobbing and Loss of Shunt. Data from this tool used the data between the wayside and the Advanced Information Management System (AIMS). The tool focuses on the hours of 8:00 AM to 9:00 AM and 4:00 PM to 5:00 PM. The report provided and sent via email includes the number of bobbing incidents per track circuit. These incidents are reviewed by signal engineers and if an investigation is required, a work order is created to instruct maintenance personnel to perform track circuit readings and current measurements flowing through the bonds to localize the issue.

Rail Replacement - Alleviate challenges related to bond saturation by either substituting the components in Traction return path or creating conditions that balance the current on the both the right and left rail. Rail replacement involves rectifying dissimilar rail issues and uneven current flow by installing matching rails on both the left and right of the track with the same restive properties. Component replacement involves the substitution of aging or malfunction track circuit elements and replacement of broken clamps and jumpers. Visual inspections are also useful in identifying and correcting loose connections.

On site investigations and troubleshooting - The remedy involves troubleshooting the LOS through data-driven maintenance practices, including the use of LOS monitoring tool. Essential steps include performing track circuit signal readings, conducting visual inspections, and implementing crucial component replacement.

• Data-Driven Maintenance: Utilize LOS monitoring tools to detect and address imminent issues.

• Visual inspection: Identify and rectify issues such as corrugated rail, arching, and broken components.

• Component replacement: Replace defective ATP components and wayside components.

This paper will propose solutions to the challenges of Track Circuit Bobbing and Loss of Shunt issues in ATC systems. The focus will be providing lessons learned that have been gained through data analyses based on calculations and testing. These targeted solutions that have been integrated have enhanced the system reliability, minimize impacts to revenue service, and continues to ensure the safe and efficient operation of trains. The proposed approach is adaptable and can be implemented in various rail transit systems to address similar challenges.

This content is only available via PDF.
You do not currently have access to this content.