When must a crew member initiate an emergency brake application?

A crew member must initiate an emergency brake application when the engineer does not respond to warnings to reduce speed. This situation highlights the importance of communication and safety in train operations. If an engineer fails to heed warnings related to speed, this can pose a serious risk to the safety of the train, its crew, and any individuals or infrastructure nearby.

In this scenario, the crew member acts decisively to mitigate a potentially dangerous situation. The emergency brake application serves as a critical fail-safe that ensures the train can be slowed or stopped rapidly to prevent accidents or derailments when other warnings are ignored. The need for such actions underlines the crew's responsibility in taking appropriate measures to maintain safety, especially in an environment where rapid response times can make a significant difference.

The other options, while they may present situations where caution is necessary, do not directly relate to the immediate need for an emergency brake application in response to a failure to act.