You will find that most stations were actually cut-and-cover, with only the Red Line having bored segments: one starting from after Farragut North [2] and running until shortly after the Medical Center [3], and another starting as soon as it leaves the CSX Metropolitan Sub northwest of Silver Spring and continues until the terminus at Glenmont, although that station itself was built from the surface [4].

Also highly recommend the book 'The Great Society Subway: A History of the Washington Metro' [5]

[1] https://www.washingtonpost.com/graphics/local/wmata-metro-de... [2] http://www.johnreilly.us/washington-metro-transit-design-and... [3] http://www.roadstothefuture.com/Metro_Glenmont_Route.html [4] http://www.nycsubway.org/wiki/Washington,_D.C. [5] https://books.google.com/books?id=beDoAwAAQBAJ