What is required to secure unattended engines or trains?

Answer: C — Apply a sufficient number of hand brakes

To secure unattended engines or trains, applying a sufficient number of hand brakes is essential. This practice ensures that the train or engines do not roll away when left unattended. Hand brakes provide a reliable means of immobilizing the train, as they can hold the train in place even on inclines. The number of hand brakes applied should be determined based on the weight and configuration of the train as well as the gradient of the track, ensuring a secure hold. Other practices, such as applying air brakes or making a full-service reduction, may engage the train's braking system but do not provide the same degree of physical security that hand brakes offer. Moreover, while notifying a supervisor might be a procedural step in some situations, it does not directly secure the train. Therefore, employing a sufficient number of hand brakes is the most effective method to ensure safety and compliance when leaving trains or engines unattended.