What does the A side isolating cock on the coupler do?

The A side isolating cock on the coupler primarily functions to isolate air to the coupler doors. This means that when the isolating cock is activated to the off position, it prevents air from reaching the coupler doors, which is essential for maintenance or when it is required to uncouple the trains without inadvertently activating the coupler mechanisms. Isolating the air supply ensures that the coupler doors remain in a controlled state and do not open or close potentially causing safety hazards or operational issues.

The other functions mentioned, like allowing air to the traction motors or extending the locking mechanism, do not align with the specific design and purpose of the A side isolating cock. Additionally, while pressure relief may be an important safety feature in general railway operations, it is not the primary role of the isolating cock in this context.