In the case of a Form D line 2 specified in both directions, when must blocking devices be applied for switches through the interlockings?

The requirement to apply blocking devices for switches through the interlockings in the case of a Form D line 2 specified in both directions is crucial for ensuring safety. Blocking devices must be applied until the Form D is either cancelled or fulfilled because this delineation maintains protection against any potential movement that could lead to accidents or conflict between trains.

For a Form D line 2, which designates specific restrictions or instructions for train movements, the need to enforce these blocking devices ensures that the operational integrity is preserved throughout the period that the Form D is active. Once the conditions set by the Form D are met or the document is officially cancelled, it is only then that the blocking devices can be safely removed.

This precaution is fundamental in railroad operations, particularly in interlocking and switch management, as it protects trains from unintended movements while acknowledging the defined operational constraints.