What must trains have to authorize reverse movements within an interlocking?

For trains to authorize reverse movements within an interlocking, they must possess a valid interlocking signal or Dispatcher permission. This requirement is in place to ensure safety and coordination in train operations. The interlocking system is designed to control train movements through an area where multiple tracks converge, helping to prevent collisions and ensure that switches are set correctly before a train can proceed.

By requiring either a valid interlocking signal or Dispatcher permission, the system ensures that the crew is made aware of the operational status of the interlocking and maintains communication with dispatch. This controls the movements of trains and clarifies that it is safe to perform a reverse movement, thus preventing any potential accidents.

The other options, while related to operational safety, do not encompass the critical aspects of authorizing reverse movements as established by the NORAC rules. The need for a signal or Dispatcher authorization is fundamental in an interlocking environment where precise control and communication are paramount.