A round (circular) yellow sign with an 'X' and the letters 'RR' warns of:

A round yellow sign marked with a large 'X' and the letters 'RR' is the advance warning for a railroad crossing ahead. It tells drivers to slow down, look and listen for a train, and be prepared to stop. The round shape is used specifically for this railroad advance-warning sign, making it distinctive. At the crossing itself, drivers may see crossbuck signs, flashing lights, and gates. Never stop on the tracks, and never try to beat a train. Recognizing the round railroad-crossing warning sign and responding by preparing to stop is important warning-sign knowledge tested on the permit exam.