A track circuit is an electrical system used in railways to detect the presence of trains on a section of track. It works by sending a low-voltage current through the rails, which creates a closed circuit when a train is present. This helps ensure safety by preventing signals from turning green when a train occupies the track. When a train enters the track circuit, it alters the electrical flow, signaling to the control system that the section is occupied. This information is crucial for managing train movements and preventing collisions, making track circuits an essential component of modern railway signaling systems.