NIR is a subsidiary of Translink, whose parent company is the Northern Ireland Transport Holding Company (NITHCo), and is one of nine publicly owned train operators in the United Kingdom, the others being Direct Rail Services, Caledonian Sleeper, Northern Trains, Transport for Wales Rail, Southeastern, LNER, ScotRail, and TransPennine Express.

[4] Suffering frequent disruption and damage to infrastructure caused by the Troubles and starved of investment by successive political administrations, the NIR network had become badly run down by the 1960s, with old rolling stock and poorly maintained track.

Despite frequent interruptions due to bomb scares, the service has remained a more or less constant feature of the NIR network.

BREL built these units between 1974 and 1977 to British Rail's Mark 2 design with some trailer cars rebuilt from hauled stock.

The sets were three-car diesel-electric multiple units, based on a more modern British design, with air-operated sliding doors.

[citation needed] Since 2002, NIR has modernised its rolling stock, with a full fleet replacement of new trains built by the Spanish company CAF.

[14] NIR also owns half of the 28 De Dietrich stock coaches used by Enterprise, the international service between Belfast and Dublin.

[20] NIR retained one Class 80 unit (three power cars and two driving trailers) as its sandite train during the 2012-2017 leaf fall seasons.

[21] However, in 2015, Translink awarded a contract to Windhoff Bahn AG to procure a new double ended multi-purpose vehicle to undertake sandite and high-pressure water spraying, as well as weed killing operations.

[25] The first units were delivered in March 2011, with entry into service in September 2011[26] In December 2018, NIR, announced that 21 additional carriages would be purchased from CAF at the cost of £50 million, via an option in the existing Class 4000 train procurement contract.

The development of railways in Northern Ireland has been linked to the future economic growth of the region, and as a way of reducing road congestion.

Upon its establishment in 1998, the Northern Ireland Assembly put in place an investment programme costing £100 million to bring about major improvements.

This saw projects including the purchase of the 3000 Class trains, the complete relaying of the Belfast-Larne line and the construction of a new maintenance depot.

This report determined that so-called "lesser used lines" were an important and economically viable part of the total network, and that investment should be consistent rather than in the "stop-go" manner of previous years.

[30] A debate in the Northern Ireland Assembly on Monday 14 May 2007[31] raised several proposals as to how the railway network could be improved: Pressure groups have advocated the protection of former routes, where the track has been lifted but the trackbed remains intact, to enable these to be reinstated for commuter traffic as an alternative to increased road building.

[32] In October 2007, following the CSR that provided funding allocation to the Northern Ireland Executive, the Department for Regional Development announced its draft budget.

Conor Murphy, the Regional Development Minister, stated that approximately £137 million could be allocated from for investment in the railways for the period 2008–2011.

[37] The pressure group Into the West, which campaigns for improved rail links to the North West region, has stated that the need for a quality rail service, as part of a larger integrated transport policy, is vital to the economic development not just for the city of Derry but for the wider cross-border region.

Once the project was completed in 2016, there is a further proposal to add two trains per day, enabling journey times between Belfast and Derry~Londonderry to be reduced by up to 30 minutes.

There have been proposals to improve the Belfast-Dublin line between Knockmore and Lurgan, enabling journey times to be reduced and frequency increased.

[35] In May 2008, the Regional Development Minister announced that his department would commission a study, in conjunction with Donegal County Council, to investigate the effects a resurrection of railway services in the north-west of Ireland with a long-term projection of building a railway line connecting Derry~Londonderry with Sligo through County Donegal.

[42] As part of NIR's original plans for its new rolling stock, it has built a new traincare depot next to Adelaide station on the site of the old freight yard.

The line is maintained both for crew training and as a diversionary route, and passes close to Belfast International Airport at Aldergrove.

[47] The reopening of the Lisburn-Antrim line is seen not simply in terms of provision of a link to the airport – it would also allow for the further economic development of the area, which has seen increases in population as people use the towns in South Antrim as dormitory settlements for Belfast.

The BMAP proposed a light rail line from the city centre that would have interchanged with a new railway station at Tillysburn, serving both the airport and the Holywood Exchange retail development.

[51][52] The pressure group Rail 21 has stated that the Tillysburn proposal is insufficient for what the new station is expected to provide – a link to the airport, transport provision for Holywood Exchange and a park and ride facility.

Due to the limited number of new units, some of NIR's older rolling stock was retained, notably the entire Class 450 on the Belfast-Larne route.

[57] In 2021, following the procurement of 21 additional vehicles purchased to extend a number of its Class 4000 units to six-cars, Translink published a new strategy indicating its intention to provide a zero emission service by 2040.