A bus driver does not need to stop when approaching:

When approaching a railroad crossing while driving a bus, you must stop between 15 and 50 feet from the nearest track to look and listen for any approaching trains. Only proceed once you are certain that there are no approaching trains. You do not need to stop at streetcar crossings; if a police officer or flagger is directing traffic; if there is a green traffic signal at the crossing; or at crossings marked "exempt" or "abandoned."
DMV Writen Test