What action must a driver take when the Critical Defect light is illuminated?

When the Critical Defect light is illuminated, the appropriate action is to travel at low speed to the next station. This procedure is critical as it ensures safety by allowing the driver to maintain control while minimizing the risk of exacerbating the defect or causing an accident. Traveling at a reduced speed provides ample time for the driver to observe any additional irregularities or issues that may arise while en route.

Other options suggest actions that could compromise safety. Traveling at high speed would increase the risk of an incident, potentially leading to a derailment or other mechanical failures. Inspecting the locomotive engine or conducting maintenance checks on doors, while important in other contexts, may not be practical or safe to address while in motion under the circumstances indicated by the Critical Defect light. Hence, gradually proceeding to the next station at a low speed is the safest and most responsible course of action.