Which sign tells you to watch out for a train?

The sign that indicates you should be alert for a train is characterized by a circular yellow shape with a black X. This design is universally recognized as a warning for railroad crossings. The circular shape signifies caution, while the black X directly represents the railroad tracks and warns drivers to look out for trains that may be crossing at that location. This sign is critical for providing advance warning to drivers, allowing them to slow down and prepare to stop if necessary.

The other signs mentioned serve different purposes. For instance, the rectangular white sign with a railroad icon typically serves as a regulatory or informative sign related to the train tracks but does not have the same immediate warning function as the yellow sign with the black X. The triangular sign that indicates a pedestrian crossing warns drivers of the potential presence of pedestrians, while the square blue sign indicating a detour provides directions for rerouting but does not relate to train crossings at all. Each sign has a distinct purpose, but the circular yellow sign with the black X is specifically designed to alert drivers to the presence of trains and the need for caution.