Happy #TravelTuesday, or #TransitTuesday (on G+)! I was in Windsor and London last week, with a couple of days to myself, so I got some sightseeing in. I stayed at a hotel in Earl's Court, which is on the District and Picadilly tube lines. The District line, being one of the older in the London Underground system, is actually above ground in many sections even close to central London. By Westminster, however, it's fully a subway.
The Westminster station puts you out right across the street from the Abbey, Big Ben, and Parliament. Horse Guards is just a walk down the street, and Trafalgar Square not much past that.