With nearly a third of Bostonians using public transit for their commute to work, Boston has the fifth-highest rate of public transit usage in the country. Boston's subway system, the Massachusetts Bay <answer>T</answer>ransportation Authority (MB<answer>T</answer>A—known as the "<answer>T</answer>") operates the oldest underground rapid transit system in the Americas, and is the fourth-busiest rapid transit system in the country, with 65.5 miles (105 km) of track on four lines. <answer>T</answer>he MB<answer>T</answer>A also operates busy bus and commuter rail networks, and water shuttles.
The MBTA is also known as the what?
T