Another issue is that because of the geography and topography of the area, if you decide to go below ground you end up having to go REALLY deep. You have to remember we are in a swamp, so you can't just go under the buildings and utility lines. You have to go deep enough that the earth won't slide or loosen due to rainfall or erosion. DC is an area where metro systems are practical, but will always be expensive to build. You also have to zone for your tunnel entry and exit assuming your previous or next station isn't underground. This is why we typically have all the underground stations in the highest density, highest property value areas along one particular line. At a certain point they become above ground because it's cheaper and not worth the cost.