Under what condition does restricted speed not apply for trains backing up?

Restricted speed is a critical safety guideline designed to ensure train operations are conducted safely, especially in situations where visibility and track conditions may be compromised. When it comes to backing up trains, restricted speed generally applies to prevent accidents and ensure that conductors have enough time to react to any obstacles that may arise.

The condition where restricted speed does not apply while backing up is when a train is passing signals indicating "Stop and Proceed." In this scenario, the train crew is aware of the signal's indication and is prepared to proceed under conditions that require caution. This means that the crew may adjust their speed appropriately as directed by the signal, allowing for controlled movement despite the backing operation.

On the other hand, other conditions provided in the choices do not override the necessity to adhere to restricted speed while backing up. For instance, backing within city limits, carrying hazardous materials, or dealing with adverse weather conditions all necessitate the application of restricted speed to ensure the safety of the train, crew, and surrounding environment. Each of these situations involves potential risks where visibility, track conditions, or proximity to populated areas may significantly increase the likelihood of an accident or danger.