<page_title> List of United States rapid transit systems by ridership </page_title> <table> <cell> 4. <col_header>  </col_header> </cell> <cell> MBTA Subway ("The T") (Blue, Orange, and Red Lines) <col_header> System </col_header> </cell> <cell> Massachusetts Bay Transportation Authority <col_header> Transit agency </col_header> </cell> </table>
The Massachusetts Bay Transportation Authority system is the fourth-busiest subway system in the United States.