What must happen if communication fails before a Form D is given a "Time Effective"?

When communication fails before a Form D is given a "Time Effective," it is crucial that trains do not proceed until communication is reestablished. This protocol ensures the safety and integrity of train operations. If the Form D, which is a crucial document for authorizing train movements, has not been made effective due to a communication failure, there is no certainty regarding the authorization for that train to proceed.

Not moving until communication is restored allows dispatchers and operators to maintain situational awareness and ensures that all trains are operating under the latest and most accurate information, which is vital for preventing accidents or unauthorized movements on the tracks. This approach prioritizes the safety of passengers and crew by eliminating any potential misunderstandings or miscommunications that may arise in the absence of real-time updates.

In the context of the other choices, stopping and waiting for further instructions can lead to unnecessary delays, while proceeding based on sight or previous instructions could introduce risks if conditions have changed since the last communication. Thus, the emphasis on reestablishing communication before proceeding ultimately reinforces safe operational practices in rail transport.