[3] Under the PPP contracts, two private consortiums (Metronet and Tube Lines) would maintain, renew and upgrade London Underground infrastructure over a period of 30 years from 2003.

[5] However, Metronet was placed in administration in 2007 after cost overruns,[8] then Transport for London (TfL) subsequently bought out the Tube Lines consortium in 2010, formally ending the PPP.

This project would include the eventual replacement of trains, new signalling and other upgrades to the Piccadilly, Central, Waterloo & City and Bakerloo lines.

[10] The ASLEF and RMT trade unions that represent drivers strongly oppose the introduction of driverless trains, saying it would affect safety.

[20][21] In 2020, a leaked TfL study found that the upgrade work required for totally unattended train operation – platform screen doors at stations and a safety walkway in tunnels[22] – would cost around £7 billion, concluding that "the financial payback is negative".

The study also indicated that automatically driven trains with a member of staff present on board (similar to the Docklands Light Railway) offered "reasonable value for money.

"[23] However, the Department for Transport pushed for the introduction of driverless trains as a precondition of any future long-term funding deal for TfL.

A TfL feasibility study showed that new generation trains and re-signalling could increase capacity: Overall, the project is estimated to cost a total of £16 billion, with a benefit/cost ratio of 4.2 to 1.

In early 2014, TfL invited train manufacturers to make expressions of interest in the Official Journal of the European Union.

[33][34] In late 2014, TfL published a shortlist of manufacturers who had expressed an interest in supplying new trains – Alstom, Siemens, Hitachi, CAF and Bombardier.

[26] During the tender period, Bombardier and Hitachi formed a joint venture (JV), and CAF chose not to submit a bid.

[42] TfL's financial issues following the COVID-19 pandemic means that the order has not been made, with Mayor Sadiq Khan requesting investment from Government.

[47] In July 2020, Prime Minister Boris Johnson visited the Goole site to mark the start of construction of the factory.