Which of the following best describes an effective locking device in relation to a switch or derail?

An effective locking device in relation to a switch or derail is best described as a lock that can only be locked or unlocked by the group that applied it. This ensures a high level of security and accountability, as only authorized personnel have access to the lock and can operate the switch or derail safely. By restricting access in this manner, it minimizes the risk of unauthorized operation, which could lead to safety hazards or accidents on the railway system.

The importance of this approach lies in the idea of controlled access—ensuring that only specific individuals or groups, typically those directly involved in maintenance or operations, can manipulate the equipment. This prevents situations where someone unfamiliar with the current track conditions or necessary safety protocols might inadvertently change settings that could endanger themselves or others.

Additionally, other options do not provide the necessary level of security. For instance, a lock that can be opened by any employee lacks specificity and may lead to unauthorized or uninformed use of the equipment. Similarly, a universal lock suggests a lack of differentiation among personnel, compromising safety and security measures. Therefore, the option highlighting the specific lock usage by the applying group aligns with effective industry practices of safety management in railway operations.