Disruption caused by the collapse of railway tunnelling works has pushed Chiltern Railways into the red, resulting in an 11 per cent dip in profits for parent company John Laing.
Chiltern, which operates services between Marylebone and Birmingham, made losses of £2.4 million for the six months to June 30, compared with profits of £4.1 million a year earlier.
Services were disrupted in July last year when earth fell onto the track at Gerrards Cross, causing the line to be closed for seven weeks.
Tesco, which was building a superstore nearby, admitted liability for the incident and Laing is pursing a claim for lost business which it is thought could be worth up to £30 million.
Chief executive Adrian Ewer, who has been in the job for nearly three months, declined to discuss the compensation range.
"It's a significant sum of money. The lasting effect of it is a loss of revenue and that is the basis of claim. It's going to take until the end of next year to settle the claim," Mr Ewer said.
PFI specialist Laing said it was still feeling the impact of passengers using other forms of transport, while it also faces reduced subsidies and cost pressures.
As a result, it expects second half losses to exceed those of the first half. Shares fell six per cent following the update.
Chiltern is still trying to win back commuters and said train performance is now at record levels with 93 per cent of services running on time. The weakness at Chiltern left Laing nursing a total drop in pretax profits to £12.3 million down from £13.8 million last year, despite seeing an improvement at its roads division.
Elsewhere, Laing has reached financial close on the £1 billion Barts and Royal London hospital project, but it said NHS budget constraints are causing UK infrastructure project delays.
As a result the group said it must start looking abroad to meet its growth aspirations and has opened offices in Canada and Europe.
The company said: "In the absence of a settlement with Tesco, the group's profits for the current year are likely to be reduced by the lasting adverse impact of the tunnel collapse on Chiltern Railways.
"That apart, we are confident of achieving a satisfactory outturn to the current year in terms of profit and portfolio value enhancement."
However, the firm admitted bid costs on projects nearly doubled to £2.3 million and were expected to rise further in the short to medium term due to the increasing level of bid activity, it said.
The company proposed an interim dividend of 1.3 pence a share, up from 1.2p.
Shares closed down 25.5 at 265.