The trains will be used on suburban services on the Great Eastern Main Line ahead of the opening of the Crossrail tunnels and the start of Elizabeth Line services in December 2018.
In the initial phase of operation from Liverpool Street TfL Rail will operate 11 trains, which have been formed as 160m-long seven-car sets to cope with shorter platforms. All 11 of these trains will be in service by September, replacing pairs of class 315 EMUs, which date from the early 1980s. Some of these trains will be retained until the full Elizabeth Line service begins in 2019.
From mid-2019 these EMUs will be reconfigured as nine-car 200m-long trains, in common with remaining 55 class 345/0 sets for Crossrail. Elizabeth Line services using full length nine-car trains will be phased in between May 2018 and December 2019 with four key stages:
• May 2018: TfL Rail service starts between Paddington Main Line and Heathrow Terminal 4, with Class 345 trains replacing the existing Heathrow Connect service and part of the Great Western inner suburban service
• December 2018: the Elizabeth Line opens between Paddington and Abbey Wood via the new Crossrail tunnels under central London, with TfL Rail-operated Liverpool Street Main Line - Shenfield plus Paddington Main Line - Heathrow Terminal 4 also operated by Class 345 trains
• May 2019: through services start on the Elizabeth Line from Shenfield to Paddington. Services are diverted from Liverpool Street Main Line station to the low-level platforms
• December 2019: the Elizabeth Line fully opens from Shenfield and Abbey Wood via central London to Reading and Heathrow Terminal 4 serving 40 stations in total.
TfL will brand services on the Crossrail network as the Elizabeth Line, named after Queen Elizabeth II, from December 2018.
A full technical description of the Aventra can be viewed here.