Translink's underestimation of the cost of upgrading the stretch of line between Derry and Coleraine by 100% is the second time the body has got its figure badly wrong.
report by John Dowdall, the Comptroller and Auditor General for Northern Ireland, examined the upgrade of the Belfast to Bangor railway line carried out by Translink in 2001.
The report showed that Translink estimated the cost to upgrade this stretch of line at £15m, but the final bill came in at £34m - more than double the estimate. The Department for Regional Development and Translink accepted the findings of the report and gave assurances that the mistakes would not be repeated.
In his report, Mr Dowdall said: "Many of the problems which arose on the project could have been avoided or their effects lessened if Translink had managed the project properly.
"Translink had no procedures for project management or the management of consultants and there were shortcomings in procurement practices."