The Rail Procurement Agency says it is confident that Dublin will get the Metro West line.
The agency has begun a series of public consultations about route options.
But the Labour Party says it is a pre-election stunt by the Government.
The agency points to the heavy demand on the M50, saying this shows the need for an orbital rail link to service Dublin's new population centres to the west.
They have two options for the over ground line. One would go through Clondalkin to link up with the planned Metro North; the other would go through Lucan.
Planners say there will be bridges over national routes like the N4 and the Liffey Valley.
The agency says Metro West will mean a journey time of 50 minutes from Tallaght to Dublin Airport.
The opposition are sceptical about whether it will go ahead, and some local residents are concerned, but planners are confident that the rail line will be completed by 2014.
