What speed must be maintained if water exceeds 3 inches above the rail?

When water exceeds 3 inches above the rail, it is imperative to maintain a speed of 5 mph. This speed is established to ensure the safety of train operations in flooded conditions. Higher speeds can lead to a greater risk of water causing instability or hydroplaning effects, potentially leading to derailments or equipment failures. By restricting the speed to 5 mph in such situations, it allows trains to navigate safely while minimizing the risk posed by the water on the tracks.

Other speed options, like 10 mph or 15 mph, would not provide the necessary caution required in flooded conditions. Additionally, a complete halt might hinder operations unnecessarily if the water level is manageable with reduced speed, making 5 mph a practical and safe compromise.