A round yellow or yellow-with-black sign with an 'X' in it means:

A round yellow sign with a black X and the letters 'RR' is the advance warning for a railroad crossing. It is placed before the actual crossing so drivers have time to slow and look for trains. The X-shaped crossbuck sign at the crossing itself is white with the words RAILROAD CROSSING in black. Federal law (and many state laws) require certain vehicles to stop at all railroad crossings even when no train is visible: school buses, vehicles carrying hazardous materials, and other commercial vehicles. All drivers should slow down and prepare to stop at any railroad crossing.