English: A London Underground S Stock unit travels north along the Metropolitan Line between Finchley Road and Kilburn. The S Stock was ordered by TfL in 2009 to replace the A Stock on the Metropolitan line, the C Stock on the Circle, District and Hammersmith and City lines and the D stock on the District line, replacing the A stock first, then the C Stock, and finally the D Stock. Currently, the S S Stock has completely replaced the A Stock on the Metropolitan line and the C stock on the Circle and Hammersmith and City lines, and is in the process of replacing the C Stock on the District line, but has yet to replace any D Stock. The process of replacing all old subsurface trains with the S Stock is to be completed by the end of 2016.