The Minister for Transport, Mr Brennan, has been told that raising the Tallaght Luas line "on stilts" over the M50's Red Cow roundabout could take up to 2½ years and cost at least €50 million.
In its response to Mr Brennan's request to consider the proposal, the Railway Procurement Agency (RPA) warned the timescale would be "significantly increased" if a new public inquiry had to be held.
The RPA said there were two options - either to delay the opening of the Tallaght line, or at least the part that would be affected, until the work was done, or to complete the line as planned and then close it later.
In a document obtained under the Freedom of Information Act, the agency notes a feasibility study by consultant engineers O'Connor, Sutton, Cronin had concluded that each of these options were "unpalatable".
The engineers, in their report to the Minister, recommended the first option, saying "seems more sensible if the design and planning of the scheme can be fast-tracked". However, they gave no estimate for delays or disruptions.
Under the plan now being implemented, the Tallaght line would be carried over the M50 on a bridge just south of the Red Cow roundabout, but it would cross two of its slip-roads "at grade", leading to fears of traffic chaos. However, the RPA, in a letter signed by its chief executive, Mr Frank Allen, said a computer simulation prepared by the Dublin Transportation Office showed trams were likely to clear the junction in a maximum of 60 seconds. Mr Allen also noted the O'Connor, Sutton, Cronin report had acknowledged that, even under the existing plan, Luas trams "can effect the crossing of the interchange without impact on traffic in normal conditions".
He said the RPA accepted the technical analysis - which was also endorsed by the consultant engineers - that Luas "would not impede movement of other traffic at the junction and would have minimal effect on road congestion".
However, the engineers had pointed out that there was such a "high degree" of congestion at the junction that "lack of discipline by motorists" in observing yellow boxes might result in delays to Luas in crossing its slip-roads.
A diagram attached to Mr Allen's letter emphasised that trams will not go through the roundabout itself, that no additional traffic lights are required or changes to their timing, which allows for up to five more trams per hour.
Turning to options for raising the line above the junction, Mr Allen warned that delaying its opening while this was done "would offer the least protection to the overall project" and would not be "an appropriate way to go about it".
As the tram depot is located west of the interchange, it would be "cut off" by the construction works, with the result that "it may not be feasible to open any part of the Tallaght line" until such time as these works were completed.
"Even with a very optimistic schedule for planning and designing the new works, construction is unlikely to begin until later than the current target completion and handover date for the Tallaght line," according to Mr Allen. He noted that Luas track work at the Red Cow "is now substantially complete", including the bridge over the M50. "There would therefore be no savings from instructing our contractors to stop track and civil works now."
As for the other option of opening the Tallaght line next summer and then interrupting it later to allow the works to proceed, Mr Allen said this would mean renegotiating the RPA's contract with Connex for operating Luas.
The total likely programme time from planning to completion would be 30 months - nine months to develop a design, three months to select a contractor, 15 months to build and an additional three months for commissioning.
But these estimates are based on a presumption that the Light Rail Order for the Tallaght line, as proposed, could be changed without a new public inquiry. If a new inquiry was needed, the timescale "would be significantly increased".
Because construction of the elevated section would have to start 450 metres from the Red Cow roundabout to provide a suitable gradient, the line would have to be taken out of service "for a period of not less than 15 months".
Mr Allen said this would result in a revenue loss of €14 million, on top of the construction cost estimated at €35 million and the value of "redundant infrastructure", such as the bridge over the M50, which has cost "at least €3 million". The RPA's analysis showed putting Luas on stilts "would do nothing to relieve road congestion at the junction". This could only be addressed through road improvements.