Which sign warns you to watch out for a train?

The sign that warns drivers to watch out for a train is the circular yellow sign with a black X. This sign is specifically used to indicate a railroad crossing, alerting drivers to the potential presence of trains and the need for caution when approaching. The design of the sign—a circular shape with a prominent black X—makes it easily recognizable and emphasizes the importance of stopping and looking for oncoming trains.

The other signs mentioned have different purposes. The diamond-shaped yellow sign with a horn symbol typically indicates a warning of a possible situation ahead, but it is not specific to trains. The rectangular yellow sign with train tracks serves to remind drivers that they are approaching railroad tracks but does not carry the same immediate warning as the circular sign. Lastly, the square blue sign with a train symbol is more likely to convey information about train services or nearby train facilities rather than serve as an active warning for train crossings.