The Downpatrick and County Down Railway (DCDR) is a 5 foot, 3 inch (1,600 mm) gauge heritage railway in County Down, Northern Ireland. It is operated by volunteers and runs passenger trains using steam and diesel locomotives, diesel railcars, and vintage carriages. The railway has approximately three miles (4.8 km) of track in a triangular-shaped layout, which connects the town of Downpatrick with the historical sites of Inch Abbey to the north and King Magnus’ Grave to the south. It also houses a museum of railway artefacts and rolling stock originating from both Northern Ireland and the Republic of Ireland, dating from the 1860s to the 1980s.

The DCDR's development was spearheaded by a group of local railway enthusiasts in the early 1980s, and work started on building the railway in 1985. Most of its track is on part of the now-closed Belfast and County Down Railway (BCDR) mainline which ran between Belfast, Downpatrick, and Newcastle. The heritage railway first opened to the public in December 1987, and originally consisted of little more than a short stretch of track with an E-class diesel locomotive and a brake van. It has expanded incrementally since then, and reached its current length with the completion of the Inch Abbey line in 2005.

The DCDR is the only five-foot, three-inch gauge heritage railway in Ireland, which has been the standard gauge on the island since the 1840s. With its Downpatrick terminus located at the foot of Down Cathedral and next to the Saint Patrick Centre, the railway has become a major tourist attraction for the town and has accumulated several hundred members. It currently has three steam and eight diesel locomotives, five of which are on loan from the Irish Traction Group, as well as the largest collection of Victorian railway carriages in Ireland. Some of these have been overhauled by the railway's volunteers from a dilapidated state, and have received awards for their restoration.

The first railway in Downpatrick was opened to the public in March 1859, with the completion of the Belfast and County Down Railway mainline from Belfast Queen's Quay. Downpatrick's railway facilities included a substantial station building on Market Street, a two-road train shed, a goods store, an engine shed, and a turntable. The line was constructed under the management of the BCDR's chief engineer Sir John Macneill, and included a bridge over the marshy estuary of the River Quoile, just outside Downpatrick. The original bridge consisted of lattice trusses supported by timber piles driven into the riverbed, and was replaced with a steel girder bridge in 1929.^{[page needed]} As it was built on the floodplain of the River Quoile, the line towards Downpatrick often suffered from flooding, particularly during the winter months.

The railway originally terminated at Downpatrick, but in 1869 a separate company, the Downpatrick, Dundrum and Newcastle Railway, built an extension from Downpatrick to the seaside town of Newcastle. The Newcastle line was operated by the BCDR and purchased by them in 1881. The extension created an inconvenience wherein a train running between Belfast and Newcastle had to stop in Downpatrick and uncouple its locomotive, which moved via a run-round loop to the other end of the train in order to continue its journey. This significantly increased journey times and caused excessive congestion at the Downpatrick terminus.

The opportunity arose in 1892 to end this cumbersome practice with the opening of the Downpatrick, Killough and Ardglass Railway and, with it, Downpatrick Loop Platform. The Ardglass line was built and operated by the BCDR, and branched off from their mainline about a kilometre south of Downpatrick station. In conjunction with this, a loop line was constructed to directly link the Newcastle and Belfast lines, with the Loop Platform located at the South Junction. This enabled Newcastle-bound trains from Belfast (and vice versa) to bypass the main Downpatrick station, and instead call at the Loop Platform where passengers intending for Downpatrick or Ardglass could board a local train. Meanwhile, the mainline train could carry on without having to run around. This resulted in an unusual triangular-shaped layout, the trackbed of which forms the basis of most of the DCDR's operational railway line.

Following the 1945 Ballymacarrett rail crash and the resultant £80,000 (the equivalent of over £3,530,000 in 2020) it had to pay in compensation, the BCDR was financially ruined. This was a deciding factor in the nationalisation of the company, which became part of the Ulster Transport Authority (UTA) in 1948. The UTA's railways were in need of significant investment following years of underfunding during World War II; as a result, Northern Ireland's devolved government established a tribunal to determine the future of its public transport system. In December 1949 the tribunal authorised the closure of all ex-BCDR lines except the Bangor branch, as part of an effort to avoid excess expenditure on railway maintenance and to divert assets towards improving the UTA's road services.

The entire Belfast – Newcastle line south of Comber, including Downpatrick, was closed on Sunday 15 January 1950. The track and almost all of the railway infrastructure was removed in 1953, and the station was used by the UTA's bus division, which became Ulsterbus in 1968. Most of the trackbed along the line was sold by the UTA to private individuals who owned adjoining land: the trackbed in the Downpatrick area was sold to Charles Mulholland, 3rd Baron Dunleath, and was left largely undisturbed until the arrival of the DCDR. Although the 1929-built Quoile Bridge was dismantled for scrap, the Loop Platform and several smaller nearby bridges were left intact. The station building was demolished shortly after Ulsterbus moved to a new depot in the town in 1975, and part of the former station yard was used as a storage depot by the Department of the Environment's Road Service.