When issuing Form D line 8 to a disabled train, which of the following actions must be taken?

When issuing Form D line 8 to a disabled train, providing flag protection to the crew is crucial for ensuring safety. This is because a disabled train may be obstructing the track and could pose a risk to other trains approaching the area. By providing flag protection, the crew of the disabled train can warn oncoming trains of the situation, reducing the risk of collisions or accidents.

Flag protection involves having a crew member equipped with a flag to signal approaching trains. This is a standard safety protocol that ensures that other train operators are aware of the disabled train's presence and can take the necessary precautions to avoid danger. It is a vital component of maintaining safety and communication on the railway during such situations.

In contrast, other options suggest actions that either do not address safety directly or could lead to increased risk. Cancelling all previous train orders might not be necessary and could cause confusion, while unrestricted movement would compromise safety. Instructing the crew to wait for further orders fails to provide immediate protective measures. Thus, ensuring flag protection aligns with the necessary safety protocols when dealing with a disabled train.