MTA stood for Metropolitan Transit Authority; its name was changed in 1964 to Massachusetts Bay Transportation Authority (MBTA). The MTA included, of course, much more than the subway; but colloquially "the MTA" meant the subway. Likewise, officially, the symbol T (in a circle) stands for all the MBTA services (it appears, e.g., on every bus-stop sign), but colloquially "the T" means the subway.