The other reason this old Capital Transit map is cool
The cool thing about this portion of the map that I have shown is that it depicts service to Rosslyn, Virginia being provided via the DC streetcar network.
This is an issue with the current "premium" transit service planning initiative underway covering the area from Union Station to Georgetown. The planning effort doesn't seem to raise a connection to the Rosslyn Metro Station as an element, even though Circulator service does dip into Rosslyn now. (Starting in 2006, I suggested that streetcar planning and Circulator service include Rosslyn.)
Back then, the DC streetcar lines, except for the 82/84 serving Rhode Island Avenue in Prince George's County and the 20 line serving the Glen Echo Amusement Park, pretty much terminated in DC. The streetcar lines serving Wisconsin Avenue and Connecticut Avenue stopped at the Maryland border. So did the line on Georgia Avenue. (A branch of this line also went to Takoma DC, ending at 4th and Butternut Streets, about one block from the old train station.)
In 1946, the Rosslyn service was provided by the 10 line, which served H Street, starting from Deanwood. According to a comment made at the H Street planning sessions in 2002, back then you could get to Rosslyn from H Street in 15 minutes by streetcar.
As various streetcar lines were abandoned, Rosslyn service was provided by different lines, including the 20 line (which went to Glen Echo) and the 80 line (which then terminated in Brookland).