When can a train resume its normal speed within a restricted area indicated by a yellow flag?

The correct choice indicates that a train can resume its normal speed once the rear of the train has cleared the restricted area that is marked by a yellow flag. This rule is in place to ensure the safety of both the train and any workers or equipment that may be within or near that area. The yellow flag serves as a warning that the area might be hazardous, and the requirement for the rear of the train to clear the zone before resuming normal speed ensures that all parts of the train are safely out of the restricted section.

This practice is critical in maintaining operational safety and preventing potential accidents that could occur if a train were to accelerate before it was entirely out of the restricted area. By waiting until the rear has cleared, it provides a buffer zone and ensures the entire train is no longer in the space designated as restricted.