What is the least likely reason for ordering a train out of service?

Ordering a train out of service is generally a serious decision that hinges on safety concerns and operational integrity. The least likely reason for taking a train out of service among the provided choices is the situation where the car is illuminated only by emergency lights.

Having emergency lights activated typically indicates a malfunction in the primary lighting system; however, it does not inherently pose a direct risk to the safety of the train operations or its passengers. While it does require attention and repair, it does not impair the train’s ability to operate safely and effectively, assuming all other systems and procedures are functioning normally.

In contrast, the other scenarios such as defective brakes, smoke detection, or substantial passenger complaints indicate immediate safety hazards or potentially important issues that could put passengers or crew at risk. For instance, defective brakes present a critical safety concern that affects stopping ability, while smoke detection can suggest fire or hazardous conditions onboard. Such issues warrant immediate attention, often necessitating the train to be taken out of service to ensure safety protocols are followed. In the case of a significant passenger complaint, depending on its nature, it could also imply safety or operational issues that require intervention.