A yellow diamond with the letters 'RR' or a railroad track symbol with an X means:

A railroad crossing warning sign (a yellow circular or diamond sign with an X and 'RR' letters, or a crossbuck X at the actual crossing) warns of train tracks ahead. Approach with caution: slow down, look both directions, listen for train horns, and be prepared to stop. At active crossings with gates and flashing lights, the gates and lights control your stop. Many crashes at railroad crossings happen because drivers underestimate train speed — a train going 60 mph looks slower than a car going 30 mph because of its size. Never try to beat a train, never stop on the tracks, and never go around lowered crossing gates. Federal law treats violations at active railroad crossings with severity.