Latest annual accounts show the rail infrastructure company at the same time lifted capital spending 15% to £4.6bn.
At 31 March 2012, Network Rail had entered into contractual commitments covering £1.7bn worth of capital expenditure, up 13% on the year before.
The company is making steady progress in delivering challenging efficiency targets set out by the rail regulator for its current funding period, which runs between 2009-2014.
Operating and maintenance costs per train mile in real terms have fallen by 84p to £7.04 during the last year – down from £12.05 at today’s prices in 2003.
Group finance director, Patrick Butcher, said: “Our results demonstrate clear and steady progress in meeting our efficiency targets. These targets are tough but we are committed to succeeding.
“In a year where Network Rail has maintained financial discipline, we have continued to deliver a larger capital programme, building the capacity of the railway of tomorrow.
Net debt rose from £25bn in 2010 to £27.3bn last year.