Final answer:
At railroad crossings marked with only a stop sign, you must stop and proceed only when it is safe to do so.
Step-by-step explanation:
At railroad crossings that are marked only with a stop sign, you must stop and then proceed only when it is safe to do so. This means that you should look both ways for any approaching trains before you continue driving. While a traffic signal would make vehicles slow down and help with pedestrian safety, if there is only a stop sign at a railroad crossing, you must use caution and ensure that it is clear to proceed.