What is the maximum speed allowed for a train after two consecutive hot boxes without defects?

The correct answer is 30 mph, which typically represents a standard speed restriction imposed following the occurrence of two consecutive hot boxes without defects. A hot box indicates that there is excessive heat generated in the wheel bearings due to friction, which can suggest issues that may not be immediately apparent even if subsequent inspections show no defects.

The significance of imposing a speed limit in this scenario is to ensure the safety of the train and its cargo, as well as to mitigate the risk of further mechanical issues that could arise from operating at higher speeds. This conservative approach is aimed at preventing potential incidents that could result from undetected faults within the train's mechanical systems. Additionally, lower speed restrictions allow for a greater margin of safety while the underlying causes of the hot boxes are being monitored or further investigated.

Other potential speed limits would not align with this precautionary principle, as they would allow for higher speeds that could increase the risk of an incident should there be an undetected issue that the crew is unaware of following the previous hot boxes. Thus, maintaining a controlled speed of 30 mph reflects a cautious response to the situation based on established safety protocols.