What is the safest assumption when a train operator receives a proceed buzzer, but the indication light remains dark?

When a train operator receives a proceed buzzer but the indication light remains dark, the safest assumption is that a side door is open. This scenario serves as a critical safety measure to prevent operations from continuing when there may be an open door, which could pose significant risks to passengers, crew, and operations. The procedure likely incorporates this assumption to ensure that the operator takes necessary precautions before proceeding, emphasizing the importance of verifying the status of all doors before moving the train.

The other situations presented in the choices do not align with standard operational safety protocols. While a signal malfunction could theoretically lead to a dark indication light, it is not the immediate assumption based on the buzzer’s alert status. Initiating emergency procedures might be too drastic unless there are clear indications of an emergency. Proceeding with caution could misinterpret the situation; without visual confirmation of clear signals and closed doors, it's vital to prioritize safety by checking the side doors before making any movement. Thus, option A reflects a standard response that prioritizes passenger safety and adherence to operational protocols.