A track circuit is a safety feature designed to detect the presence of trains on a railroad track. This circuit relays information to automated control systems, as well as human controllers, and is used to increase safety on train tracks. Circuits are placed at numerous points along a track and are designed with failsafes so that in the event there is a problem, unsafe conditions are not created.
When a train is present, it creates a short in the track circuit. This illuminates an indicator light and sends a signal to tell controllers that a train is present and provide information about the location. For automated control systems, track circuits are used to send signals to tell trains to slow down, stop, or engage in other activities. The control logic used to send remote signals to the train can also make decisions about which trains to move where on the basis of the number of trains present, their speed, and other factors.
Human train controllers rely on track circuits and other devices to tell them where trains are. In addition, if a human controller sends a signal that is clearly unsafe, like a message to proceed when a train is already on the tracks, the system can override that signal and prevent the message from being sent. This greatly reduces the risk that a human error will cause a train accident.
As a failsafe, if a track circuit is not working, it will usually display an indicator showing that a train is present. In the short term, this allows the control logic and manual controllers to proceed as though a train is present, the most ideal situation for safety. In the long term, the continuous indicator that a train is on the tracks serves as an alert that the circuit needs to be examined and repaired. Some defective circuits may also do what is known as “bobbling,” toggling back and forth rapidly between indicators showing the tracks are clear, and indicators showing the tracks are occupied.
The introduction of the track circuit to railway design has significantly increased railway safety, in addition to enabling the development of more sophisticated automated control systems. Train accidents still occur despite the best efforts of the industry, and on occasion are linked to damaged or malfunctioning track circuits that sent the wrong signals. Using layers of safety devices helps avoid these situations; if a single track circuit fails, for example, another down the line may still function and provide a warning.