Network Rail, which runs Britain's railway tracks and signals, is set to ask the government for an extra £7bn.
The firm says the extra money is needed to increase capacity
The firm wants the money to invest in infrastructure on busy commuter routes to cope with rising passenger numbers.
Passenger numbers are set to rise by a third over the next decade and extra money is needed to extend stations and put in new lines, says the firm.
The £7bn is in addition to the £20bn it is already tipped to request for the period from 2009 to 2014.
The operator will ask for the money from the Office of Rail Regulation amid wider funding talks with the government.
The news comes after the firm recently posted annual pre-tax losses of £232m, up from £47m in 2005.
The Department for Transport was recently reported to be considering letting train operators offset increasing demand by increasing passenger fares.
Network Rail instead favours making the necessary investments to improve the network, to cope with rising passenger numbers and reduce overcrowding.
The firm says commuter lines such as those into London and Birmingham need new sections to cope with demand.
Network Rail receives about £5bn to run its network at present, but says this does not allow for improvements.
The operator says it should be able to operate on £4bn at a future date, but this is still at the higher end of the Office of Rail Regulation's estimates.
The firm's latest losses, which were posted in May, were largely down to paying interest on money it had borrowed to fix the railway network.
On releasing its results, Network Rail said train punctuality had reached at a six-year high, and £3bn had been invested in the network, including 700 miles of new track.
The firm, whose top executives are due to receive big bonuses, hopes to be profitable this financial year.