The "Loopline bridge" is a heavy railroad bridge that crosses the Liffey just west of the Custom House. It was built in 1891 to connect train lines on the north and south sides of Dublin, blocking views of the large, handsome neoclassical structure. After crossing the river it stays elevated on both the south and north banks, passing over many city streets.
This prominent Dublin feature is first mentioned in Lotus Eaters, when the elegant woman whom Bloom has been ogling, waiting for her to climb up on a jaunting car and reveal glimpses of her underwear, rides off "towards the Loop Line bridge." The "bridge" here is quite a few blocks from the water, passing over Great Brunswick Street on its way to the nearby Westland Row (now Pearse) railway station, where it terminates.
One chapter later, in Hades, the carriage in which Bloom is riding passes "under the railway bridge" near Westland Row station. In the space between the two chapters Bloom has traveled southeast to Paddy Dignam's house in Sandymount, and is now revisiting the area he walked through before.
The bridge proper (that is, the part that crosses the river) figures in Wandering Rocks, when the throwaway which Bloom has thrown away on the O'Connell Street bridge drifts past its pilings: "A skiff, a crumpled throwaway, Elijah is coming, rode lightly down the Liffey, under Loopline bridge, shooting the rapids where water chafed around the bridgepiers, sailing eastward past hulls and anchorchains, between the Customhouse old dock and George's quay." In 1904 oceangoing ships moored both upstream and downstream of the Loop Line bridge, navigating past the immediately adjacent Butt Bridge by virtue of its swiveling central section.
The action of Eumaeus takes place under the northern part of the bridge, behind the Custom House. Stephen and Bloom, we hear, "made a beeline across the back of the Customhouse and passed under the Loop Line bridge where a brazier of coke burning in front of a sentrybox or something like one attracted their rather lagging footsteps." This is the post of Gumley, guarding the Corporation's stones.
From the nearby cabman's shelter, Bloom sees the old sailor Murphy "gaping up at the piers and girders of the Loop line rather out of his depth as of course it was all radically altered since his last visit and greatly improved." Bloom, then, is much in favor of this recent feat of mechanical engineering in service of railway connectivity, accomplished during Murphy's long absence. He does not seem to be bothered by any aesthetic violence it may have done to the Georgian cityscape.
At its northern end, the bridge runs into the Amiens Street (now Connolly) station, the departure point for northbound trains. This station too is mentioned in Eumaeus and Ithaca.