For a Form D line 2 in a specified direction, how long must blocking devices be applied to switches through the interlockings?

The requirement for blocking devices applied to switches through interlockings in the context of Form D line 2 emphasizes safety and coordination in train movements. The correct answer indicates that blocking devices must remain in place until the train crew is certain that the train clearance from the interlocking has been established. This ensures that no conflicting movements can occur while a train is passing through potentially hazardous areas.

By keeping the blocking devices applied until it is known that the track is clear, the operational integrity and safety protocols are upheld, minimizing the risk of accidents. This approach reflects a proactive safety measure, ensuring that miscommunication or errors regarding train location do not lead to dangerous situations.

The other options may suggest different duration or conditions for releasing blocking devices, but they do not align with the need for absolute certainty of the train's position relative to the interlocking, which is the primary consideration in ensuring safe operations.