The 77A is a short-turn trip of the 77 Arlington Heights - Harvard Station via Massachusetts Ave. bus. It simply exits the north end of the tunnel onto Massachusetts Avenue, which it uses to the loop at the North Cambridge Carhouse. At the Harvard end, it turns around in the same way as the 72. However, these loops are not normally made, as since January 2005, the only 77A trips are pull-ins and pull-outs to take 71 and 73 buses to or from the carhouse. They are listed in the 77 schedule.
September 4, 1958 was the last day of streetcar service on the 77A (and 71 and 73), after which trackless trolleys have run. Until around 1967, the route was numbered 82.
Read more about this topic: Boston-area Trackless Trolleys