The Trans-Dublin is a railway route serving the city of Dublin and its surrounding areas. The route is used primarily by the DART service, and runs north-south along the east coast of Ireland. The route is divided into two parts over two different railway lines:
- Belfast to Dublin main line - This runs from Dublin Connolly to Belfast Central. The northern section of the Trans-Dublin operates between Dublin Connolly and Howth Junction, where it branches off. The main line continues to Malahide, while a short branch terminates at Howth.
- Dublin to Rosslare main line - This runs from Dublin Connolly to Rosslare Europort. The southern half of the Trans-Dublin runs as far as Greystones in County Wicklow.
There is a currently shelved proposal to construct an Interconnector railway tunnel under the centre of the city to connect Dublin Heuston, Dublin Pearse and Docklands. This would be used to create two "new" DART lines. The northern section will be diverted away from Dublin Connolly to Docklands and then through the new tunnel calling at new underground stations at Dublin Pearse, St Stephen's Green, High Street and Dublin Heuston, where it will connect with the lines to the south and west of Ireland, terminating at Hazelhatch. The southern section will continue on its existing route as far as Dublin Connolly, where it will then be diverted via the Western Commuter route via Drumcondra to terminate at either Maynooth, or the new Pace M3 Park and Ride station on the re-opened Navan branch.