Network Rail, the company which runs everything on Britain's railways apart from the trains, saw its debts soar by almost £1bn to more than £28bn in the past six months.
Commuters may not be surprised to discover that the infrastructure group admitted it had missed punctuality targets.
Network Rail said that £2.06bn investment in lines and stations – including redeveloping Reading station and works on the East Coast and West Coast main lines – saw net borrowings rise from £27.2bn in March to £28.04bn in September.
Network Rail's finance director, Patrick Butcher, played down concerns about the debt pile.
"We need to borrow to invest to make the railway bigger, because more and more people want to use it," he said.
The business receives a taxpayer subsidy worth £3.5bn a year to look after railway tracks and signals, with its debt guaranteed by the Government.
Mr Butcher said: "Our asset base is £45bn compared to a debt of £28bn. If you look at any other utility business, like National Grid or Thames Water, it's a fairly typical ratio. Providing our asset values keep going up – and they are, because they are driven by growing income streams from more passengers – then high debt levels are perfectly acceptable."
Network Rail, which is a not-for-dividend company and has no shareholders, said it was also hit by the increase in the valuation of RPI-linked bonds, after it raised £1.5bn this year by issuing six new sterling and dollar bonds. It said the value of the railway network rose from £43.1bn at the end of March to £45.3bn at the end of September.