Final answer:
The main reason for releasing the stop arm again after it has been hooked down to pass an automatic signal is to ensure the safety of the train and its passengers.
Step-by-step explanation:
The main reason for releasing the stop arm again after it has been hooked down to pass an automatic signal is to ensure the safety of the train and its passengers. By releasing the stop arm, the train operator allows the signal to return to its normal position and communicate with other trains on the track. This helps to avoid confusion and potential collisions between trains.