The Phoenix Park Railway Tunnel is a Victorian engineering achievement from 1877, running under Phoenix Park to connect the south and west of the city without disturbing the parkland above. Passengers on commuter rail services passing through experience a dramatic minute of complete darkness — an unexpected sensation in an urban train.
The tunnel is not open to visitors, but taking the train through it (on services between Connolly and Heuston via the Phoenix Park line) is a worthwhile quirky experience, and the tunnel portals near Islandbridge are visible from the riverside walk.