<page_title> List of United States rapid transit systems by ridership </page_title> <table> <cell> 4. <col_header>  </col_header> </cell> <cell> Massachusetts Bay Transportation Authority <col_header> Transit agency </col_header> </cell> <cell> Boston <col_header> City/Area served </col_header> </cell> </table>
Boston's public transportation agency, the Massachusetts Bay Transportation Authority (MBTA) is the fourth-busiest rapid transit system.