What is a likely cause of a dark train operator's indication?

A dark indication for a train operator typically suggests a lack of clear information or communication from the signaling system. In this context, one likely cause is that the side door is open. When a train's side door is open, it can prevent the signaling system from displaying the correct indicators, which can lead to a dark indication being produced. This is often due to safety protocols in place that require confirmation of all doors being closed before the system will allow for normal operational signals.

The other potential causes, while plausible in certain scenarios, do not directly correlate to a dark indication in the same way. A power outage could indeed affect signaling but would usually lead to different types of alerts indicating a systemic failure. Signal failure can result in a lack of clear aspects, but the direct link to a dark indication is less straightforward compared to an open door situation. Similarly, while a switch malfunction could cause operational issues, it would not typically be indicated as dark without further encompassing signs. Therefore, an open side door is a more logical explanation for a dark indication experienced by a train operator.