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Alstom Audio Frequency Track Circuit Technology: Innovation in Urban Transit Train Control
Authors: Leandro Aveiro, Brandon Gaskin, and Howard Goldberg
We’ve all done it – living or visiting a major metropolitan area and riding their subway or transit system to get from one place to another. We expect it to be safe, on-time and even enjoyable. The primary function of a transit signaling system is to safely move as many people as possible. Accomplishing this lofty goal requires trains to travel as close to each other as possible (called “headway”) while maintaining a safe and smooth ride. To do this, the signaling system must “know” where trains are at all times. The track circuit, or more specifically the Audio Frequency Track Circuit (AFTC), is a subsystem used for this purpose in urban transit applications.
What is a track circuit and why are they needed? There are many different types of track circuits in use today. A track circuit utilizes the track rails as conductors in an electrical circuit between transmit and receive devices, which define the limits of the track circuit. When the solid, electrically conductive wheels and axles of a train roll into a track circuit or if a rail is cracked or broken, the signal between the transmit and receive devices is interrupted and the system declares 16 | The ROCHESTER ENGINEER NOVEMBER 2022
the presence of a train in that track circuit. Depending on the application, track circuits can employ electrical signals that are simple DC (direct current), pulsed DC, low frequency AC (60Hz, 100Hz), or Audio Frequencies (1KHz to 10KHz). A key feature of track circuits is their ability to detect rail defects and breaks. Although there are competing technology alternatives to track circuits that use radiobased communication and devices called axle-counters to detect train position, these technologies do not use the rails as a medium for signaling and therefore they cannot detect rail defects like track circuits can. Rails experience punishing stresses from trains, exposure to seasonal temperature extremes, and daily thermal cycling, which all contribute to cracking over time. Detection of rail defects and breaks is key to safely operating trains and is a tangible benefit to transit operators in avoiding derailments. Newer track circuit technologies, like the Alstom AFTC Gen 5 discussed later, are capable of analyzing rail signals and reporting potential rail defects before they become hazardous while preventing trains from entering areas with suspected broken rails. cover article - Alstom