When should PKO by-pass be utilized?

The PKO by-pass should be utilized specifically when there is an inability to obtain a brake release light. This situation indicates a malfunction or issue within the brake system that prevents the normal operation of the air brakes, which is a critical component for train safety and control. By bypassing the PKO (Pneumatic Keep-Out) system in this scenario, the crew can ensure that they are still able to operate the train safely despite the malfunction, allowing for necessary actions to be taken to mitigate safety risks.

In contrast, other contexts such as being on a scheduled route, the crew’s discretion, or during emergencies do not inherently warrant the use of the PKO by-pass. In standard operations or scheduled routes, the system is expected to function as designed. Using the by-pass simply because the crew deems it necessary, without a specific failure prompting it, could compromise safety. Similarly, while emergencies require prompt responses, they do not automatically justify bypassing unless a specific brake release issue is present. Thus, the correct answer emphasizes a direct operational protocol linked to a clear technical issue.