Several routes branch out from the tunnel along routings where installation of overhead trolley wire would have been impractical, especially through the Ted Williams Tunnel (as overhead wires are expressly prohibited on interstate highways), so the Waterfront section uses dual-mode buses. The vehicles run as trolleybuses within the tunnel and for about on the surface, then change to diesel power at the end of Silver Line Way, and reverse the transition when inbound. Motor buses are not permitted to operate inside the South Boston Waterfront tunnel due to insufficient ventilation. Four routes run in the electrified Silver Line tunnel: SL1 to Logan Airport, SL2 to Design Center, SL3 to Chelsea, and a shuttle route which short turns at Silver Line Way. Until February 2006, the Silver Line also used 12 of the then-new conventional-length Neoplan trackless trolleys until the entire articulated, dual-mode fleet could be placed into service; they were temporarily "wrapped" in the Silver Line's silver paint scheme, covering the standard MBTA yellow-and-white.<ref name="tm260"/ Only three of the 32 dual-mode buses on order had been accepted for service by the time the Silver Line opened, but the number had increased to 28 by February 2006. In fall 2005 (five vehicles) and the first half of 2006, the 12 Neoplan trackless trolleys were transferred to the Harvard-based routes to replace the then-remaining Flyer trolleybuses operating out of North Cambridge Carhouse. Fleet Past fleet Boston's first trolleybuses were built by Pullman-Standard in 1936. Boston Elevated continued to buy its trolleybuses almost exclusively from Pullman, and until 1950 Pullmans comprised the entire fleet with the exception of a small batch of five Twin Coach trolleybuses purchased in 1937. In 1952, Boston had more Pullman-built trolleybuses than any other city, with 438. Meanwhile, 25 ACF-Brill trolleybuses built in 1950 were the only other purchase of non-Pullman trolleybuses until 1976. With several route closures in 1964, the Twin Coaches and the oldest Pullmans were retired, leaving only Pullmans built in 1947–51 in service. These were all replaced in 1976–77 by a fleet of 50 Flyer E800s (4000–4049), the first of which arrived in February 1976 and entered service in April. The last Pullman-Standard trolleybuses were retired in August 1977. The Flyers were, in turn, replaced by new trolleybuses over the period 2004–06. Regular use of Flyers ended in June 2006, but a small number remained active and used very sporadically until March 2007. The