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17 January 2017, 12:12
The company, that won the contract to run Greater Anglia again last summer, says the joint venture should lead to "significant improvements" for passengers.
The announcement comes on the same day Abellio Greater Anglia came in the bottom five in an annual survey of customer satisfaction on the railways by consumer group Which?.
It ranked as one of the lowest - along with strike-hit Southern Rail, Govia Thameslink and Great Northern, South West Trains and Southeastern - for punctuality, reliability and value for money.
A statement from Abellio has said: "Ourselves and Mitsui have a proven track record of working together, having first entered into a joint venture to bid for the West Midlands rail franchise in 2016, along with East Japan Railway Company.
"Following this process, Abellio felt that Mitsui would be the best partner to help it deliver its ambitious programme to transform the Greater Anglia franchise."
It's not gone down well with the RMT Union, who've said Britain's rail network is "constantly up-for-grabs," and it makes a "mockery" of the Government's franchising process.