How long are Form D's in effect?

Form D's remain in effect until they are either fulfilled or canceled because they are specific instructions issued for train movements and activities. This means that the conditions and permissions outlined in a Form D are active and enforceable until the objectives stated in the form have been completed or until someone actively cancels them. This allows flexibility in operation and ensures that train movements can safely proceed until the necessary conditions have been met or the directives have changed.

Understanding this is crucial for dispatchers as it highlights the importance of monitoring the execution of train operations and making timely decisions to ensure safety and efficiency on the rail network. The other options suggest arbitrary time limits that do not reflect the operational nature of Form D's, which are intended to be effective based on operational needs rather than a fixed time frame.