Following the implementation of the London Underground Public Private Partnership (PPP) in 2003, the Metronet consortium became responsible for the infrastructure on the District, Circle, Hammersmith & City and Metropolitan lines.[3] As part of the PPP, Metronet would deliver 190 new trains built by Bombardier Transportation and install new automatic signalling from Westinghouse Rail Systems. The first train was planned to enter service on the Metropolitan line by 2009, with all trains in service by 2015.[3] All the trains would be built to the same design, saving on parts and maintenance costs for Metronet.
In July 2007, Metronet, the private consortium responsible for the infrastructure for the sub-surfaces lines, collapsed due to financial difficulties.[4] TfL subsequently took over the contract for the new trains, and organised a new contract for the replacement of signalling.
In 2011, a £350m contract was awarded to Bombardier to replace the signals on the four lines with their Cityflo 650 system.[5] This work would be completed by 2018.[5] However the contract was terminated by TfL in 2014 due to delays, cost overruns and the complexity of the task.[6][7] The decision by TfL to pay Bombardier £80m to end the contract was subsequently criticised by the London Assembly.[8]
In 2015, the contract was awarded to Thales, with the replacement of signalling now costing £760m.[6][9] It would also be delivered four years later than originally planned, in 2023.[6][10]
The order was for a total of 192 trains (1,403 cars), and formed of two types, S7 Stock for the Circle, District and Hammersmith & City lines and S8 Stock for the Metropolitan line. The main differences, aside from the number of cars (S7 having seven cars and the S8 having eight cars), were in the seating arrangements, in which the S7 Stock consisted of a longitudinal-only layout, whereas the S8 Stock had a mixture of longitudinal and transverse seating. New features that were not used on the previous rolling stocks included air-conditioning, low floors to ease accessibility, and open gangways between carriages.[11][12]
Part of the modernisation includes the introduction of Communications-Based Train Control (CBTC) to allow for Automatic Train Operation (ATO) via Thales's SelTrac system.[13] In order to upgrade the signalling, Signal Migration Areas have been created to allow for the gradual installation.[14] Below is a list of the SMAs and their progress:
As a result of SMA 5 being installed, the Circle line began running entirely under ATO and after the completion of SMA 6 the Hammersmith & City line also now runs completely under ATO.
For various reasons including funding and the technical difficulties with sharing tracks with National Rail & the Piccadilly line, SMAs 10-12 were scaled back until further notice.
