I just stumbled on this image over at DCDIG which shows the flooding in Georgetown on Mar 19, 1936. The Georgetown Branch ran from the right side to the bottom, through the arch of the old Aqueduct bridge and toward the photographer. If you look carefully you can see the old cast iron DUAL telltales in the foreground! They span the right of way and warned brakemen of the approaching arch. Remember, brakemen rode the roofs of cars back in the older days. The Potomac Boat Club boathouse survives to this day, as does the arch of the Aqueduct bridge.