The Phoenix Park Rail Tunnel will open to regular passenger services for the first time next week.
The 690 metre long tunnel was built in 1877; however it has never been used for regular passenger trains until now.
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The line will open from next Monday and will introduce new services linking Newbridge and Hazelhatch in Kildare with Connolly Station and Grand Canal Dock in Dublin.
Existing services from Newbridge and Hazelhatch to Heuston will continue - although, as things stand, the service will not connect Heuston and Connolly.
The Phoenix Park Tunnel upgrade came at a cost of €13.7m, funded by the National Transport Authority.
The timetable for the new Phoenix Park route is available on the Irish Rail website.