S Stock rollout

The S Stock rollout was a project in the 2010s to replace the old sub-surface stocks: A Stock, C Stock, and D Stock. It started back in 2010, when the S8 Stock on the Metropolitan line entered service, and in 2012, the last of the A Stock fleet was completely replaced. This event followed, in 2014, when the C Stock was fully replaced. The project was originally due to be completed in 2015, but it was delayed by 2 years, and wasn't finalized until 2017, when the D Stock was replaced, though the S7 Stock was introduced to the line in 2013. The last of the D Stock ran in service for the last time on 21 April 2017.