Which condition is considered a safety hazard while trailing with cars?

In the context of trailing with cars, a condition like both ditch lights not functioning correctly is indeed a significant safety hazard. Ditch lights serve an important role in enhancing a train's visibility, particularly during low-light conditions or in inclement weather. Their absence could make it difficult for other vehicles or personnel to see the train, increasing the risk of accidents and jeopardizing safety on the tracks.

On the other hand, while excessive speed and the movement of indicators may raise concerns, they do not directly impact the visibility of the train in the same way that non-functional ditch lights would. Similarly, having only one inoperative horn might affect communication signals but is not as critical as the visibility issue presented by the ditch lights. A fully functional brake shoe is also not a safety hazard; instead, it contributes positively to the braking system's effectiveness.