Historical version 5 of Hammersmith And City Line (view current version)
Formerly regarded as part of the Metropolitan Line, but formally separated in 1990, the H&C runs between Hammersmith Station in the west and Barking Station in the east; outside peak hours, eastbound trains reverse at [Plaistow Station]?, [Whitechapel Station]? or even Moorgate Station or Edgware Road Station.
The inimitable Clive Feather has a page on the history and layout of the line.