Abellio has agreed to sell 40% of its Greater Anglia rail franchise to Japanese firm Mitsui.
The line has been run by Greater Anglia since 2012 and the company renewed the franchise in August.
It says the deal will result in a £1.4bn investment over nine years, with new trains and average journey times reduced by 10%.
The rail union RMT said Britain’s rail network was being sold off like it was a “dodgy car boot sale”.
Mick Cash, general secretary of the RMT, said the deal, which follows the sale of the c2c franchise to Ternitalia last week, was “making a mockery” of the Department for Transport’s (DfT) franchising process.
“The checks and balances for both passengers and the taxpayer, which the DfT claims are enshrined in its multi-million pound franchising programme, are clearly lacking when the winning bidder can simply walk away, share out its responsibilities and choose its replacement whenever it sees fit,” he said.
“They are flogging off Britain’s transport assets like Derek Trotter at some dodgy car boot sale.”
Abellio has refused to reveal the value of the deal, which is pending final regulatory approval.
Dominic Booth, managing director of Abellio, said: “With the introduction of Mitsui’s knowledge and experience, we look forward to delivering significant improvements for Greater Anglia’s customers.”
Mitsui has yet to comment on the deal.
Last year both companies put in a joint bid to run the West Midlands franchise. A decision on who will operate that franchise is expected from the DfT in June.
A DfT spokeswoman said: “This sale is a commercial decision for Abellio. The government will only approve this partial sale once both parties have satisfied us that passengers will benefit from it.”