The Fallout 3 Metro is a term used to describe a series of metro tunnels, stations, and other locations that are connected to or have a relation ship with a metro of some kind in Fallout 3. There are a good amount of metro's in the game, most of them are crumbled and caved in because of the 200 years exposed to the elements, and a lot of them have enemies like Feral Ghouls and Raiders inside.
A map of the metro's of DC itself is shown below, note that not all Fallout 3 Metro's will be seen here. Click for closer view, or alternatively click this link for the bigger picture.
This is a list of these said metro's, and they were organized in alphabetical order. The list goes up to down, and then up again.