Irish Rail is warning that work on a DART line linking the proposed DART Underground with an overground line to Dublin Airport needs to happen before land around it is built on.
Planning permission for the link was granted back in the mid 2000s but lapsed during the recession.
Yesterday a committment was given to DART Underground in the government's National Planning Framework.
However a link between the airport and Clongriffin DART station is only under consideration.
Irish Rail's Barry Kenny says that if the proposed line is built on, the option will be gone forever.
"Effectively you can operate it to Clongriffin and then as a normal DART in or as an express, but our concern is that alignment be protected".
"If that alignment is lost, it's lost forever", he warned.