1972 map of the MBTA Green Line (including the Watertown Line, then believed to only be temporarily suspended) with the "D" Branch highlighted
Transportation System Center, USDOT
<a rel="nofollow" class="external text" href="https://archive.org/stream/mbtagreenlinetes00neat#page/n0/mode/2up">MBTA Green Line Tests: Riverside Lines, December 1972</a>