A Stormont minister has rejected claims his department has "too cosy" a relationship with Translink after the estimated cost for the Coleraine to Londonderry rail line doubled.
Regional Development Minister Danny Kennedy told the committee of MLAs that monitors his department that Translink's costing had been a "guesstimate".
He said the reasons for the original bill of £20m-£22m shooting up to £40m would be examined by Translink chiefs, who would report back by the end of the year.
Quizzed by Ukip MLA David McNarry, Mr Kennedy characterised his department's links with the company as "healthy and professional", and said no one "had been given cover".
Asked whether heads might roll, the minister insisted he would not predetermine the outcome of the review. It will involve chief operating officer Philip O'Neill, who revealed design costs had increased because of changes to accommodate bigger trains.
SDLP MLA John Dallat said it was crucial the long-planned upgrade did not become a victim of Stormont cuts.
"The Executive must be mindful that this railway line is now a vital tool in developing the economic and social infrastructure of the north west. It will in the future be a critical part of Ireland's network of railways from Derry to Kerry," he added.
Speaking afterwards, Mr McNarry said: "It is clear that both the department and Translink are stuck up a dark tunnel not having a clue what the cost will end up being for the rail track.
"Will heads roll? They should, but I doubt it. It's a disgrace."