What is the LZB signaling system, one of the key factors in the fatal train accident in Spain?

The LZB signaling system, used on the Madrid-Seville high-speed line, monitors train speeds and transmits instructions to drivers. A recent collision may have resulted from a failure in this system, leading to manual control and a subsequent crash. The line is transitioning to the more advanced ERTMS system, which enhances safety through radio communication. Various safety mechanisms, including ASFA and block-control systems, are in place to prevent accidents. Investigators are examining the sequence of events to identify any errors that contributed to the incident.