The Government has given initial approval for spending of around €1 billion as part of a plan to double passenger capacity on Dublin's DART network.
The DART + programme also involves trebling the length of track to 150km in the greater Dublin Area.
The first phase will be DART West, which involves the electrification of the Maynooth line, and this has been given approval to enter the planning system.
The cost of this has been estimated at up to €1.1 billion.
The Government has agreed to give a "preliminary business case" for four other parts of the plan.
They are DART + Fleet, which involves the drawdown of up to 750 new carriages, DART + South West, which is the electrification the Kildare line through the Phoenix Tunnel to Hazelhatch, DART + Coastal North, which is electrification to Drogheda, and DART + Coastal South, which involves improvements to the existing Greystones line.
The programme is being funded as part of the National Development Plan.
Minister for Transport Eamon Ryan said: "This is a hugely significant decision which fulfils one of our key public transport commitments in the Programme for Government.
"This is a tangible demonstration of progress, which will transform public transport and help us meet our climate goals."
When completed the DART + is due to double the number of passengers an hour to 52,000.
It is planned that permission for a railway order for DART + West will be sought from An Bord Pleanála in 2022, and that construction will take around four years.
National Transport Authority CEO Anne Graham said DART + will be a "game changer" in providing an alternative to the private car and reducing emissions.