What is the main reason for releasing the automatic stop arm after passing an automatic signal?

Releasing the automatic stop arm after passing an automatic signal is primarily done to give protection to a following train. When a train passes a signal, it is crucial to ensure that subsequent trains are aware of the current status and conditions on the line ahead. By releasing the stop arm, the system communicates readiness for the following train to proceed safely, thereby preventing potential collisions or misunderstandings regarding track occupancy. This action plays a vital role in maintaining the overall safety and efficiency of rail operations, ensuring that each train can navigate the network with appropriate awareness of the conditions ahead.

While the other options may touch on important procedures in train operations, they do not capture the fundamental necessity for releasing the stop arm in relation to the safety and signaling context specifically aimed at protecting following trains.