Under what condition can an engineer operate a train in door bypass?

Operating a train in door bypass is permissible specifically when the doors are confirmed to be free from obstructions or encumbrances. This ensures the safety of passengers and crew members, as it reduces the risk of accidents or injuries that could arise from doors being inadvertently opened while the train is in motion. Clear doors allow the engineer to operate the train without concern that the doors will open unexpectedly, thus maintaining operational integrity and safety standards in rail operations.

The other options would not provide the same level of assurance or operational feasibility required for door bypass. For instance, operating only in emergencies could lead to situations where safety protocols are compromised. Conducting repairs while in bypass could create hazardous conditions if repairs are not complete or if resources are limited. Lastly, relying solely on notification from the conductor without verifying the state of the doors does not prioritize the safety measures necessary to operate the train effectively in door bypass.