Translink has come in for scathing criticism over delays and overspends in the re-laying of the Belfast to Bangor railway line.
The upgrade took place on the Bangor-Belfast line
An Audit Office report said "serious management deficiencies" led to the 2001 project costing £34m - twice the budget - and running nine months late.
It said design problems meant that train speeds are restricted to 70mph.
The transport company said the recommendations in the report had been accepted and were "being acted upon".
The Belfast to Bangor line carries about two million passengers a year and, in January 2001, the Department for Regional Development approved a project to completely re-lay the line.
However, the Audit Office pointed to a "very significant overspend" in the project, which, it said, was as a result of "failures" in several key areas.
These included "serious deficiencies" in the economic appraisal which "under-estimated" the cost of the project.
The Audit Office also pointed to problems with Translink's lead engineering consultants, who, it said, "failed to deliver" designs for the track.
These failures, it said, contributed to multi-million pound claims from the construction contractor.
The overall cost to taxpayers was said to be in the region of between £8m and £13m.
The report added: "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."
It also said that Translink's documented record was "very poor" adding that "key decisions were not recorded and documents were destroyed".
The Audit Office also said there were "indications of excessive generosity to Translink staff".
The report concluded that Translink had made progress with plans to improve project management but implementation in some areas "has been slow".
It said the department had acknowledged that "significant lessons have been learned".
In a statement, the Northern Ireland Transport Holding Company said it would not comment in detail until the Public Accounts Committee had considered the report.
However, it added that the project had been "beset with major difficulties" and the report had "identified a number of recommendations to address weaknesses and prevent a recurrence of such events".
"These recommendations have been accepted by NITHCo/Translink and are being acted upon, as demonstrated by the introduction of new procedures for the management of infrastructure projects," it said.