Irish Rail says the alignment for a connection between the Dart and Dublin Airport must be protected.
Yesterday a commitment was given to extend the in the Government's National Planning Framework.
However, a link between the airport and Clongriffin Dart station is only under consideration.
Corporate Communications Manager with Irish Rail, Barry Kenny, says that if the proposed line is built on, it will cease to be an option.
"Effectively you can operate it to Clongriffin and then as a normal Dart in [to Dublin city centre] or you can operate express if preferred," he said.
"Ultimately what our concern is that alignment be protected because if that alignment is lost, it's lost forever."
Irish Rail has also dismissed reports that most of the network faces being closed.
It had been claimed some intercity routes faced the axe unless there was a significant increase in investment.
However, the company has moved to reassure passengers that is not the case.
It follows the publication of a report on the funding needs of the firm.
"The rail review did not conclude that we close the majority of the national network," said Kenny.
"The rail review said the options here are do we have the existing network fully funded by the exchequer or do we do a mix of additional funding and look at some lightly used lines.
"So not the radial network, not your Westports and Sligos and Galways to Dublin."