A Class B system is a national railway control or protection system that is recognized separately from the European Train Control System (ETCS) in EU interoperability rules. These legacy systems may use country-specific equipment and operating rules. LZB is one example.
Class B systems matter because trains and infrastructure may need to manage transitions between legacy protection and ETCS, or operate under special procedures when protection is degraded. In practice, operators define system boundaries, speed limits, authorization steps, and fallback procedures to keep movement controlled. Accurate status information and records help crews act safely and support later audits. The term describes a category of railway system; it does not by itself indicate a fault, cyberattack, or particular line configuration.