@ozdreaming If it's from the mid-1950s then presumably the North Cambridge bus routes were still streetcar routes at the time. (There is a long and sad story of the MTA and then MBTA cannibalizing all of its old streetcar lines to free up rolling stock to run the Riverside line, culminating with the closure of the Watertown line in the late 1960s. IIRC, Blue Hill Ave. was dieselized to move trolleybuses to North Cambridge to replace the streetcars that were moved to Riverside.)