When must blocking and/or marking devices be changed in relation to original track and time limits?

The correct answer highlights the importance of safety and communication in train operations, particularly within a dispatching context. When changing track and time limits, it is crucial to ensure that blocking and marking devices are not removed until new limits are adequately protected. This ensures there is clear and uninterrupted communication about which tracks are occupied, which are clear, and where trains may safely operate.

By keeping the blocking and marking devices in place until the new limits are established and protected, it reduces the risk of accidents or confusion that may arise from overlapping limits. This procedure guarantees that all personnel involved are aware of the current conditions on the tracks and helps maintain a safe environment for train operations.

Without this precaution, there could be instances where trains could inadvertently operate on tracks that are not properly cleared, leading to potentially dangerous situations. The emphasis on protection before removal aligns with best practices in rail safety and operational integrity.