According to SRA chairman Sir Alistair Morton, private capital could make up to £34bn of the £60bn investment needed for the railways. He admitted that £34bn was an "aspirational" figure that could not be confirmed.
Railtrack would continue to direct maintenance and renewals business. Firms that already have an interest in rail maintenance, including Laing, Balfour Beatty and Bechtel, have been tipped to take interest in the partnership schemes.
Railtrack chief executive Steve Marshall said the company accepted that it could not develop the rail network on its own.
Schemes set to go ahead under the investment plan include upgrades on the East Coast, West Coast, Great Western and Midlands main lines.