Good point! In fact, they are doing it again this year which means on 7/4 (and only on that day), blue line trains terminate at Rosslyn whereby everyone gets off and transfers to orange line trains that snake through downtown (i.e. blue line trains will not be allowed into the city). In that case, it may be easier to take the yellow line from DCA to L'Enfant Plaza and switching to the orange line to get to McPherson Sq. At L'Enfant Plaza, go down the escalators and take any orange line train in the direction of Vienna. Either way, echoing the previous poster, make sure you look out for the actual station you want (McPherson Sq) instead of the colors of the Metro line.