When can an alternate marker, like a red flag, be used to indicate the end of a train?

The use of an alternate marker, such as a red flag, to indicate the end of a train is typically employed when a highly visible marker cannot be displayed. This situation may arise under specific conditions where the standard end-of-train marker, which is often a lighted device, is not available or visible, possibly due to environmental factors or equipment failure. Using an alternate marker ensures that the end of the train is still clearly indicated to oncoming trains and other personnel, maintaining safety and communication on the tracks.

In contrast, the other options suggest scenarios that do not align with the guidelines for using an alternate marker. For instance, a malfunctioning engine whistle does not directly relate to the visibility of the train's end. Operating in yard limits often has different rules and might not necessitate the same signaling protocols as mainline operations. Lastly, the idea that an alternate marker could be used under any circumstances does not reflect the established safety regulations that govern train operation and signaling. These regulations are in place to ensure clarity and prevent accidents, requiring specific conditions to be met for alternative signaling methods to be employed.