Footnote 5 http://ghostsofdc.org/2013/10/01/never-built-metros-bridge-rock-creek/ at this link https://en.wikipedia.org/wiki/Taft_Bridge has some interesting info about the Metro engineers' original plan to have the tunnel at Rock Creek be somewhat shallower and come outdoors and actually PIERCE the columns of the Taft bridge. So the subway trains would emerge from the canyon wall, travel along the Taft bridge but underneath the main roadway, and go back into the canyon wall on the other end. This would have allowed the tunnel to stay a bit more shallow.

And as a Metro user, that would have been SO COOL.

But the important factor here was that the National Park Service, the federales who run Rock Creek Park, said hell no. So all the stations that serve the West side of the Red Line had to burrow that much deeper to get under the Rock Creek gorge.