Yikes. I thought this died. And now I feel kind of bad for letting this drag on.
If all of the people are still up for it - you're all accepted. Make sure to say something in this thread, and I'll start work on a post.
so If this isn't dead, I noticed you didn't appear to have a few things in the OP. Some stuff you may find useful:
http://upload.wikimedia.org/wikipedia/commons/3/37/WMATA_system_map.svg
http://en.wikipedia.org/wiki/File:Washington_DC_Metro_Map_%28To_Scale%29.svg
http://www.wmata.com/rail/maps/map.cfm
Now for some other stuff:
Arlington Cemetery is an above-ground station. The Yellow line has an above-ground bridge crossing the potomac. The Green-line, I think that has a bridge as well between Navy Yard and Anacostia. Once your out of D.C, Most of the southern half (that's outside D.C) of the Green line is above ground stations with a mixture of tunnels and bridges. In fact, once you leave D.C itself, the Metro system starts making use of above-ground stations and bridges.
Other tidbits:
The D.C armory is near the Stadium-armory station.
The Smithsonian metro is right on the National Mall, right by the museums.
Metro's basically consist of two floors, an upper platform where you can put money on a ticket stub, and a lower floor where the train tracks actually are. (This upper area may be in the same chamber as the tracks or not, depends on which station your in) Also, most of the metros have an elevator to the surface. Tracks may also be in the middle of the chamber or at the sides. Depends on what station your in.
A couple of the metro's, like pentagon city and Crystal city, connect to larger complexes. Specifically, a mall in Pentagon city's case; and a mostly underground series of tunnels, with shops inside and office buildings accessible by elevators and the like in crystal city's case.
http://soulofamerica.com/soagalleries/dc/trans/DC-Metro_train.jpg
edit: also, one thing I've noticed is that none of you specified what stations you live in. Where you start would have the biggest impact on where you could go in the metro system.