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The Belfast–Dublin line is a main international railway route on the island of Ireland that connects Dublin Connolly station in the Republic of Ireland and Belfast Central station in Northern Ireland.
The railway line was built by three separate companies. In 1837 the Ulster Railway began building a railway line between Belfast and Lisburn, which was extended in stages to Portadown in 1842 and as far as Clones by 1863. The Dublin and Drogheda Railway (D&D) built the line between Dublin and Drogheda. The Dublin and Belfast Junction Railway (D&B Jct) linked the Dublin and Drogheda with the Ulster Railway at Portadown. The D&D and the D&B Jct merged in 1875 to form the Northern Railway of Ireland. In 1876 this new company merged with the Ulster Railway and the Irish North Western Railway, forming the Great Northern Railway (Ireland) (GNRI).
The partition of Ireland in 1922 meant that the Irish border passed between Newry and Dundalk, which caused lengthy delays as trains were required to stop at stations on either side of the border for customs examinations. This disruption was eased in 1947 with the opening of facilities for customs checks at Amiens Street station and Great Victoria Street station.
At the same time, the GNRI made its Belfast-Dublin services non-stop with the launch of the Enterprise Express. The GNRI was nationalised by the governments of the Republic of Ireland and Northern Ireland in 1953 as the Great Northern Railway Board, but in 1958 this was split between the Ulster Transport Authority and Córas Iompair Éireann. This led to a running down of rail services in Northern Ireland, leaving only some Belfast commuter lines, the northern route to Derry and the link to Dublin. In 1970 the newly formed NI Railways bought new locomotives and rolling stock for the Belfast-Dublin Enterprise service as well as new diesel multiple units for local services.
In the year 2000, the government of the Republic of Ireland developed a National Development Plan, which has seen major investment in infrastructure. Almost the entire railway network, including the Belfast–Dublin line as far as the border, has been upgraded to Continuous Welded Rail, while signalling is controlled using the Centralised Traffic Control system located at Dublin Connolly station.
In addition, in 1997, a set of new De Dietrich Stock coaches were purchased jointly by Northern Ireland Railways and Iarnród Éireann to operate a revamped Enterprise service along with the new Class 201 locomotives.
In addition to the inter-city service between Belfast and Dublin, both NIR and IÉ operate local services along the route. NIR operates local services along the northern half of the line (the Belfast–Newry line) between Belfast and Lisburn, Portadown and Newry, while IE operates its Commuter services between Dublin and Dundalk as part of the Dublin Suburban Rail network. In addition, the line between Dublin Connolly and Malahide is electrified and forms part of the Trans-Dublin DART network.
The line is also used by rail passengers changing at Dublin Connolly onto the DART and also by connecting bus travelling to Dublin Port for the Irish Ferries or Stena Line to Holyhead and then by train along the North Wales Coast Line to London Euston and other destinations in England and Wales.
Holyhead can also be reached by Irish Ferries from Dublin Port, reached by walking beside the tram lines around the corner from Amiens Street into Store Street or by Luas four stops to Dublin Port or Dublin Bus route 53  or to take a taxi.
The route has been released as a commercial add on for Microsoft Train Simulator by Making Tracks. It was released in two sections, part one covering Belfast Central to Dundalk, with part two covering the section from Dundalk to Dublin. It is set during the 2000s.
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