An Bord Pleanála has expressed concern that the proposed Luas link-up in Dublin city centre would interfere with future development of the DART Underground and Metro North projects.
Construction of the LUAS link line BXD was originally scheduled to take place after completion of the two underground lines.
The board points out that the LUAS line will be above three planned Metro stations and one for the DART Underground.
In a letter to the Rail Procurement Agency (RPA), the board states it is not clear how future works can be carried out while keeping the BXD line open.
It would also have reservations about any proposal to suspend or replace this Luas service.
The Metro North and Underground DART projects have been given the go-ahead by An Bord Pleanála.
But these plans were shelved in the Government's spending review last November because of the combined cost estimated at €5bn.
The Government said it would go-ahead with BXD, which is estimated to cost €300m.
An oral hearing on the BXD application was completed before the Government announced the suspension of the underground projects.
A decision on BXD was expected soon, but in its request for further information the board warns that any modification of existing plans would need fresh approvals.
It also wants the National Transport Authority to approve the response of the RPA.
A spokesman for the RPA said it was confident that the concerns of the board could be addressed. It has until the end of March to respond.
