What allows two trains to occupy the same manual block?

The correct answer highlights the specific circumstances under which two trains can safely occupy the same manual block. The provision of allowing this is contingent upon verbal communication, as dictated by part-241, and the control exercised by the block operator. This mechanism is crucial because it ensures that both trains are aware of each other's presence and that necessary precautions are taken to maintain safety.

Verbal communication ensures real-time updates, allowing for dynamic adjustments based on the situation. The block operator's control facilitates the coordination between the two trains, providing oversight to avoid collisions and ensuring that operational protocols are followed.

Other options imply alternative methods or rules that do not align with safety practices. For example, suggesting that a written dispatch order alone enables two trains to occupy the same block undermines the need for immediate communication and situational awareness. Similarly, stating that only one train can always be present contradicts the premise of the question regarding shared use under controlled circumstances. Lastly, the idea that a single train can simply allow another to enter suggests a passive approach to a process that requires active management to ensure safety compliance.