The Green Line is a streetcar system run by the Massachusetts Bay Transportation Authority (MBTA) in the Boston, Massachusetts area of the United States. It is the oldest line of Boston's subway, which is known locally as the 'T'. The Green Line runs underground downtown and on the surface in outlying areas. With a daily weekday ridership of 237,700, it is also the most heavily-used light rail line in the country. The line was given the green color because it goes primarily though an area called the Emerald Necklace of Boston. The four branches are the remnants of a once large system of streetcar lines, begun in 1856 with the Cambridge Horse Railroad. The Tremont Street Subway carries cars of all branches under downtown, and is the oldest subway tunnel in North America, opened in stages between September 1, 1897 and September 3, 1898 to take streetcars off surface streets.