Subway system in
Washington,
DC and surrounding parts of
Maryland and
Northern Virginia, operated by the
Washington Metropolitan Area Transit Authority.
One of the cleanest subway systems in the world, probably due to strict bans on food and drink. Relatively expensive, but worth it to avoid driving in the area. Doesn't go as far out into NoVA as one would like (Dulles International Airport and Tysons Corner would be appreciated), but expansion is on the docket as soon as a spare billion can be found.
Fares are charged on a paper-based magnetic stripe card by number of miles you travel and time of day (it's more expensive at rush hour). This is different from most other subway systems I've seen, where a single fare will get you anywhere on the system, but it seems to work, partially due to heavy discounts for commuters.