The Massachusetts Bay Transportation Authority (MBTA) is "a boy politic and corporate, and a political subdivision" of the Commonwealth of Massachusetts formed in 1964 to finance and operate most bus, subway, commuter rail and ferry systems in the greater Boston, Massachusetts, area. It replaced the Metropolitan Transit Authority (MTA), immortalized by the Kingston Trio in the popular folk-protest lament "M.T.A.". Locals call it simply "The T", after its logo, the letter T in a circle, adopted in the 1960s. In 2006, the system averaged 1.1 million passenger trips each weekday, of which the subway averaged 598,200, making it the fourth busiest subway system in the United States. The Green Line and Ashmont–Mattapan High Speed Line of the T comprise the busiest light-rail system in the U.S, with a weekday ridership of 255,100. The MBTA also operates its own independent law enforcement agency, the Massachusetts Bay Transportation Authority Police.