What must be true for a dispatcher to allow a train to proceed under Rule 556?

For a dispatcher to allow a train to proceed under Rule 556, it is essential that the block be unoccupied by another train. This rule is primarily concerned with ensuring the safety of train movements. When a block is occupied, the risk of collision significantly increases, making it crucial for the dispatcher to verify that the block is clear before granting permission for a train to enter.

This safety measure ensures that there is sufficient space for the train to operate without the danger of encountering another train on the same track section. Other factors, while potentially relevant to operational procedures, do not directly pertain to the fundamental requirement for safe train movement defined by this specific rule. For instance, while it is beneficial for cab signals to be operational and for the dispatcher to communicate clearly, the primary concern remains the occupancy status of the block in question.