The Boston-area trolleybus (or, as known locally, trackless trolley) system forms part of the public transportation network serving Greater Boston in the U.S. state of Massachusetts. It opened on April 11, 1936, and since 1964 has been operated by the Massachusetts Bay Transportation Authority (MBTA). The current system consists of the Silver Line (Waterfront), a bus rapid transit service using dual-mode buses which run as trolleybuses in a tunnel in the Seaport District of Boston before switching to diesel power to serve other destinations. Until 2022, several other trolleybus routes fanned out from the Harvard bus tunnel at Harvard Square station, running through Cambridge, Belmont, and Watertown. These lines were the remains of a once-extensive system of trackless trolleys in the area, which was largely formed from former streetcar lines. Measured by fleet size, the Boston-area system was the third-largest trolleybus system in the United States at its peak (end of 1952), with only the Chicago and Atlanta systems having more trolleybuses than Boston's 463. Aside from four routes feeding the Harvard tunnel, Greater Boston's trolleybus routes had all been converted to diesel bus operation by 1964. History The first trackless trolley line in the Boston transit system was opened by the Boston Elevated Railway (BERy) on April 11, 1936. Replacing a streetcar line over the same route, it was a crosstown line (later numbered 77, and today served by the bus) running from Harvard station east to Lechmere station. (Substitution of buses for streetcars on the route had been proposed as early as 1930.) Additional lines were opened in 1937, and by 1942, the system had 14 lines, of which 10 were former streetcar lines and four were former motor bus lines. In 1947, the BERy was succeeded by the public Metropolitan Transit Authority (MTA) as the operator of Boston's urban transit system, and in 1964, the MTA was replaced by the Massachusetts Bay Transportation Authority (MBTA), which remains the system's operator today. The BERy was owned by private investors, but came under public control from 1918. It served 13 municipalities of Greater Boston, including Boston proper. The MTA's service area was limited to these municipalities (with the addition of Revere). Following a change of management, the MTA began replacing trolleybuses with motorbuses, starting in 1958. Only four trolleybus lines remained after 1963, all based in Cambridge. Four decades later, the MTA's successor, the MBTA, built the Silver Line