Irish Rail is calling for a link to be made between the airport and Dublin city.
The Irish Examiner reported that a commitment was given to the Dart Underground project in the government's National Planning Framework yesterday.
However, connecting Dublin airport to Clongriffin Dart station is only under consideration.
Irish Rail said that, although it is only being considered, the proposed line for the link needs to be protected.
Corporate Communications Manager with Irish Rail, Barry Kenny said: "Effectively you can operate it to Clongriffin and then as a normal Dart in [to Dublin city] or you can operate express if preferred."
"Ultimately what our concern is that alignment be protected because if that alignment is lost, it's lost forever."