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The O2 logo is seen as England's George Ford kicks a penalty during their international rugby test match against Samoa at Twickenham in London, Nov 22, 2014. [Photo/Agencies]

Eversholt owns around 28 percent of the UK's passenger trains, with 19 fleets. It is one of the UK's three leading rail rolling stock companies.

3i, Morgan Stanley Infrastructure Partners, and STAR Capital Partners with its co-investor PGGM, acquired the business for 2.1 billion pounds from HSBC in 2010.

Unlike most private equity funds, which usually hold investments for between four and six years, infrastructure funds generally hold their assets for longer periods of time as they are seen as steady, low-risk investments.

"While 3i Infrastructure generally aims to hold its Core investments over the long term, it will sell investments from time to time where this generates significant additional value for shareholders," said Peter Sedgwick, Chairman at 3i Infrastructure.

"It is in this context that we have taken the decision to sell our shareholding in Eversholt Rail."

The sale of 3i's stake will generate around 358 million pounds, it said, as well as a 15.5 million pound sum received in December. A source close to the deal said that STAR, a London investment fund manager, would make a return of 3.4 times its investment.

Another source familiar with the matter said that CKI pre-empted an auction process for Eversholt after losing out on the sale last year of Porterbrook, another UK rail rolling stock company.

Eversholt, Angel Trains and Porterbrook have dominated the market since 1994 when the UK government privatized the country's railways. The companies have proved attractive to investors for their steady income, longer-term leases and the country's growing numbers of rail passengers.