When can blocking devices be removed when delivering a Form D?

The correct understanding in this situation is that blocking devices can be removed after the engineer acknowledges receipt of the Form D. This ensures that the train crew is fully aware of the instructions and limitations imposed by the Form D, which is crucial for safety and operational efficiency.

By requiring acknowledgment from the engineer, it establishes a clear communication line, confirming that they are prepared to proceed with the information provided. This acknowledgment acts as a safeguard, ensuring that all operational protocols are understood and that the necessary precautions are in place before moving on to the next operational step.

Other possible options, such as removing blocking devices immediately upon issuing the Form D or after the dispatcher confirms delivery, lack the necessary checks and balances that acknowledgment from the engineer provides. Additionally, waiting until the train is ready to depart could lead to confusion or unintentional movement before all parties have fully understood the situation, making the engineer's acknowledgment a crucial step in this process.