When is it permissible to place a wedge under the vestibule door or lock the latch of the vestibule doors to keep the door open?

The correct answer relates to safety protocols that guide the handling of vestibule doors on trains. Placing a wedge under the vestibule door or locking the latch to keep the door open is typically permissible when there is a failure of the air conditioning system. In such cases, keeping the vestibule doors open can help maintain airflow and provide a more comfortable environment for passengers, especially during hot conditions.

The other scenarios presented do not align with safety practices established for train operations. For instance, during regular maintenance, the emphasis is usually on ensuring that all systems are functioning correctly, which would not involve propping doors open. Similarly, crowding on a train is generally managed through proper station procedures rather than making adjustments to the vestibule doors. Lastly, taking instructions from a passenger to prop a door open is not standard procedure and could lead to safety hazards, as decisions about door management should be made by trained personnel based on operational guidelines.