English: C stock train and new S7 stock train at Hammersmith Station, July 2013. Following the withdrawal of the A stock on the Metropolitan line in 2012, the C stock became the oldest stock in service on the London Underground. The S stock consists of the S8 stock and the S7 stock. The S8 stock was introduced in 2010 to replace the aging A Stock on the Metropolitan line, which reached 50 years of service in 2011 and at the time of its final withdrawal was the oldest stock in service on any rapid transit system in Europe. The S7 stock was introduced in 2012, making it the newest stock in service on the London Underground network. The S7 stock was ordered to replace the C stock on the Circle, District and Hammersmith and City lines, and the D stock on the District line. The C stock was completely replaced in 2014, and the S7 stock is to replace the D stock by 2016.