When must crew members act if the engineer fails to comply with a signal indication?

In the situation where the engineer does not comply with a signal indication, crew members have a responsibility to ensure the safety of the train operation and the people around it. The correct course of action is to use the emergency brake if necessary. This action is critical because it allows the crew to immediately address a potentially dangerous situation. A failure to obey a signal can indicate a serious risk, and promptly applying the emergency brake can help prevent an accident or collision.

Taking immediate action is crucial in these circumstances. The safety of passengers, crew, and infrastructure depends on quick decision-making and effective response to any deviations from established protocols. Using the emergency brake serves as a direct method to halt the train's movement and prevent further escalation of the situation.

In contrast, notifying the public immediately or consulting with the train manager may lead to delays in addressing the issue and could compromise safety. Waiting for the next scheduled stop would not be appropriate, as it would not mitigate the risk posed by the engineer’s actions. Therefore, ensuring the train comes to an immediate stop through the use of emergency measures is the most effective and responsible option.