Transport for London (TfL) is seeking suppliers for the procurement of a new fleet comprising 39 four-carriage electric trains.
They will have similar features to the existing London Overground fleet, including walk-through carriages, air-conditioning and improved accessibility.
Beginning in 2017, 30 will run on the existing West Anglia route and will replace the 28 trains that are currently operating.
Eight of them will be used for the electrification of the existing Overground route between Gospel Oak and Barking, while the remaining one train will run on the line connecting Romford and Upminster.
TfL rail director Jonathan Fox said: "We are inviting train manufacturers worldwide to do business with us.
"Our customers have already seen huge improvements to the existing London Overground network and we will be bringing that same expertise and those high standards to a wider group of passengers and regular London commuters."
The West Anglia inner services between Liverpool Street Station and Enfield Town, Cheshunt (via Seven Sisters) and Chingford, come under TfL management in May 2015 and will be operated as part of the London Overground network.
The Gospel Oak to Barking route is being electrified by Network Rail, with work expected to be completed in 2017.
TfL also intends to upgrade 24 stations with a deep clean, new signage, improved CCTV and better passenger information systems.
Image: London Overground Turbostar DMU 170005 approaches Leytonstone High Road with a service from Barking to Gospel Oak. Photo: courtesy of mattbuck.