The British Rail Class 345 Aventra is a fleet of electric multiple unit passenger trains built by Bombardier Transportation for use on London's Elizabeth line.

In 2008, the British government's rolling stock plan stated a requirement for around 610 carriages for Crossrail, expected to be similar in design to the Thameslink rolling stock, to meet the design improvement requirements of the 2007 "Rail Technical Strategy" (RTS), including in-cab signalling/communication with satellite and European Train Control System level 3 technologies, regenerative braking, low cost of operation and high reliability, with low weight and high acceleration.

[11][12] The publicly released specifications included a passenger capacity of 1,500, with 450 seated, in a fully air-conditioned train no longer than 205 m (672 ft 7 in) with a top speed of 145 km/h (90 mph), and an energy efficiency as good as 24 kWh per train-km.

[15] The total value may be greater due to the winning bidder being expected to undertake maintenance of the trains for three decades, the estimated lifespan of the fleet.

[18][19] In August 2011, the invitation to tender was delayed by one year to 2012 and the contract decision to 2014, with the introduction of trains on the Great Eastern Main Line expected from May 2017 (previously December 2016), with a correspondingly shortened production schedule.

The delay was a cost-saving measure to avoid new vehicles being unused whilst Crossrail tunnelling was completed;[20] it also postponed bidding until after a review of governmental procurement processes.

[23] Concerns about taxpayer value for money on PFI funded projects led to Transport for London (TfL) seeking to purchase the trains outright.

[24][25] In December 2011, the request to raise the debt ceiling at TfL to allow the acquisition with public funds was refused by the Department for Transport (DfT).

[47] The first train entered service on 22 June 2017 on the eastern TfL Rail route between London Liverpool Street and Shenfield as a seven-car unit.

[48] The trains entered service on the western TfL Rail route between London Paddington and Hayes & Harlington in May 2018, before running to Reading by December 2019.