D.C. Metro Guide
Al, I rode the D.C Metro extensively in March, on a weekly pass which paid off handsomely.
Joel (his name's not John) is correct: Fares are based on distance, much like the San Francisco Bay Area's BART.
Check out the site for the fine print, and possible cost-saving measures.
Oh yeah, and if you say ROSS-lyn, you'll be quickly corrected. The locals call it ROZ-lyn, like Jimmy Carter's wife.
And be prepared to see uniformed military personnel aboard. They're not guarding the trains, just commuting to and from their work, usually at the Pentagon. In 2004 I found myself standing next to a Navy Captain in full dress whites, and involuntarily saluted. Didn't have to; just reflex from working for the Navy decades ago. We struck up a conversation, and he was a very pleasant guy.
And in March I saw the same Army sergeant - smartly attired in battle fatigues, and carrying a briefcase - almost every day. Just happened to be doing the commute at the same time.
And Al, don't be surprised to see crowds of people carrying signs in stations. D.C. is a government town, after all, and that's a magnet for protesters.
In '04 I was surrounded by a pack of anti-choice demonstrators led by a guy ordering them around with a bullhorn. Just take it in stride; part of the deal.