Ireland in the 1880s was poorer than London understood. The Conservative government of Lord Salisbury, with Arthur James Balfour serving as Chief Secretary for Ireland from 1887 to 1891, decided the answer was infrastructure - and specifically, railways to places that ordinary commercial railway companies would never build. Balfour pushed the Light Railways (Ireland) Act through Parliament in 1889, providing state funding for branch lines into economically underdeveloped regions. The Act's lines became known as Balfour Lines. Most ran into the rural west of Ireland - Donegal, Kerry, Connemara. Ardglass was the one Balfour Line built in Ulster, and the case for it rested entirely on herring. The County Down fishing fleet had been growing fast through the 1880s, and Ardglass Harbour was the busiest landing port on the east Irish coast. What it lacked was a way to move the catch inland fast enough to reach Belfast markets before the fish spoiled.

The Downpatrick, Killough and Ardglass Railway Act passed in 1890. The Belfast and County Down Railway absorbed the new branch into its existing system, and the BCDR's chief engineer Sir John Benjamin Macneill - one of the most accomplished Irish civil engineers of the Victorian era, a former pupil of Thomas Telford - took on the construction. The line ran eight miles from Downpatrick South Junction south to Ardglass, with intermediate stations at Ballynoe, Killough, and the Downpatrick Racecourse. Construction was completed in time for freight services to start on 31 May 1892. Passenger services followed quickly, on 8 July of the same year. A short tramway extended from the end of the railway line through Ardglass village - along Downpatrick Road, Bath Street, and Quay Street - to a point near the northeast end of Ardglass Harbour Pier, where the fish was loaded directly from boats to trains.

For thirty years the herring business sustained the line. Boats landed catches in Ardglass; trains carried them north to Downpatrick, then on through the BCDR network to Belfast and beyond. Local passenger traffic was secondary - a useful supplement, never the main business. The single-track route was initially divided into two operational segments managed by the staff-and-ticket system, with signal boxes at intervals to manage train movements safely on the single line. Two later halts were added for local convenience: Bright Halt opened on 12 October 1925, and Coney Island Halt on the coast in 1929, named after the small offshore island visible from the line. The tramway link to Ardglass Harbour Pier itself, however, was already falling out of use by the mid-1920s as motor lorries took over the short-haul movement of fish. In 1926 the Ballynoe signal box was decommissioned, consolidating the line into a single operational section. The Killough signal box, which had never actually functioned as a block post, closed at the same time.

Two forces ended the line. The herring fishery itself, which had built Ardglass into a port of national reputation, began a slow contraction in the years between the world wars as overfishing depleted the Irish Sea stocks and as the centre of British fishing shifted northward to Scotland. The other force was the motor lorry, which by the 1930s could move fish from Ardglass harbour to Belfast as fast as the train and without the awkwardness of two transfers. The Belfast and County Down Railway itself, the parent company, struggled financially through the depression and post-war years. Passenger numbers on the Ardglass branch dwindled. The station closed to passengers in 1950, by which time it had been taken over by the newly-formed Ulster Transport Authority - the public body that absorbed Northern Ireland's bankrupt private railways. Freight services ended not long after. The tramway to the harbour was lifted. The fishing industry that justified the line outlasted the line by decades, but never again on the scale that had once required a branch railway.

The station buildings still stand at Ardglass, mostly roofless and derelict as of recent surveys. The trackbed has been lifted and parts of the route are now footpaths or have been built over. Ardglass itself remains a working fishing port, smaller than it was at its Victorian peak but still active - prawn boats and scallop dredgers replacing the herring fleet that built the railway. The harbour pier where the tramway once delivered fish wagons is now mainly used by yachts. Down the coast at Killough, the village that the Balfour Line served halfway along its route, the small Georgian streetscape remains largely intact, with little to indicate that trains once ran along the back of the village. Walking the route today - past Ballynoe, past Killough, into the outskirts of Ardglass - is to walk a piece of Ulster industrial history that was always anomalous: the only state-subsidised light railway in what would become Northern Ireland, built for a fishing trade that had already begun to slip away from the County Down coast before the rails were even finished rusting.