(Waterfront), and that line's opening in late 2004 reintroduced trolleybus service in Boston proper. At the trolleybus system's peak, the maximum number of trolleybus routes in operation was 37, with most route running in the near north suburbs of Boston. In addition, there were the disconnected Dorchester and Arborway networks. Prior to the opening of the Silver Line, a total of 43 trolleybus lines had historically existed, but not all concurrently, making the all-time total 44 (counting the Silver Line trunk as one route) . Table of former trolleybus routes Proposed system closure In 2022, the Silver Line (Waterfront) is planned to be converted from dual-mode buses to diesel hybrid buses, with the overhead wires taken down afterwards. Following the testing of a small number of battery buses on the line starting in July 2019, MBTA placed an order in November 2020 for 45 articulated "extended-range" diesel hybrid buses to replace the 32 dual-mode buses currently used on the Silver Line. MBTA staff also recommended replacing all trolleybuses on the Cambridge-based network with battery buses. Trolleybus service on the last two surviving routes, 71 and 73, ended on March 12, 2022. Multiple utility and road-rebuilding projects beginning in early 2022 have led MBTA to decide to substitute diesel hybrid buses for trolleybuses at that time. The move was initially temporary, but if the authority later approves the proposed conversion of the trolleybus system to battery buses, the temporary suspension would become a permanent closure. Harvard-based routes History What became the Harvard-based trolleybus system began as branches of the Cambridge Railroad, a horsecar street railway that opened in 1856. Lines from Harvard Square to Mount Auburn via Brattle Street and Porter Square opened soon after the Harvard Square–Bowdoin Square route. The connecting Watertown Horse Railroad opened on April 27, 1857. The Porter Square branch was extended to the border of West Cambridge (now Arlington); there it met the West Cambridge Horse Railroad, which opened on June 13, 1859. In 1880, the Watertown line was extended south on Galen Street to Nonantum Square. The Harvard–Arlington line was electrified on July 1, 1889 – one of the first lines converted by the West End Street Railway. Electric service between Watertown and Mount Auburn began on December 12, 1893; the line was double-tracked later in the decade. Residents of Brattle Street objected to the construction of electric lines; in October 1893, the Board of