What action should be taken if a train needs to slow down quickly?

Engaging the emergency brakes is the appropriate action to take if a train needs to slow down quickly. Emergency brakes are designed to provide maximum braking force and facilitate a rapid decrease in speed, effectively bringing the train to a stop as swiftly and safely as possible. This system is crucial for maintaining safety under critical conditions, such as when an obstruction is detected on the tracks or when the train is in danger of exceeding speed limits.

Other actions, while they may have their own purposes, do not directly address the necessity for an immediate and powerful response to halt the train. For instance, continuously sounding the horn may alert individuals nearby but does not contribute to slowing down the train. Notifying passengers is important for communication but does not influence the train's speed directly. Switching off the power supply could lead to uncontrolled braking in some situations, but it is not a standard method for quickly reducing speed and can create additional hazards. Thus, engaging the emergency brakes is the most effective and standard response in this scenario.