The Belfast–Derry line (referred to as the Derry~Belfast Line by NI Railways^[2]) runs from Belfast to Derry in Northern Ireland. The line is double-track on the short section it shares with the Belfast-Larne railway line, but is composed primarily of single track from Monkstown to Derry with passing points at Templepatrick, Antrim, Magherabeg, Ballymena, Killagan, Ballymoney, Coleraine and Bellarena.

Weekday and Saturday services on the line run hourly from Great Victoria Street in Belfast to Derry and vice versa (except for arrivals in Belfast after 11pm on weekdays which terminate at Lanyon Place). Hourly shuttle services to and from Portrush connect with the line at Coleraine.

On Sundays services run hourly from Great Victoria Street to Derry, but only every two hours from Derry to Belfast. During those hours when trains do not depart from Derry, the shuttle service from Portrush to Coleraine continues on to Great Victoria Street. This means that only passengers departing from Derry, Bellarena and Castlerock have a two hour wait between departing services. The service arriving in Belfast after 10pm terminates at Lanyon Place.

On all days the earliest/latest services in each direction begin/terminate at Portrush or Coleraine.

Prior to 2001 and the reopening of the Bleach Green viaduct, services operated via Crumlin, Glenavy, Ballinderry and Lisburn. The reopening of the Bleach Green viaduct resulted in shorter journeys between Belfast and Derry. A skeleton service continued on the Lisburn–Antrim line until 2003, when the line and its stations were closed. This section of railway is now used solely for driver training or other operational requirements e.g. special services to major events.

In August 2011 it was planned to reduce services on the Coleraine to Derry section to five services, in each direction on weekdays, to facilitate safety improvement works during 2012. A refurbishment of the line was due to commence in April 2012, however the £75 million that it was to cost, was unavailable. This resulted in opposition from supporters of the section who feared that the line would be permanently closed down.^[3]

In October 2011, after years of uncertainty, Regional Development minister Danny Kennedy relocated funding from the A5 dualling project to the railway upgrade project, allowing for a 3-phase upgrade, which commenced in July 2012.

Phase 1 saw the line close for nine months to completely relay two sections (Coleraine to Castlerock; and Eglinton to Derry) of the route, extending the life of the remaining section by converting the currently jointed track to continuous welded rail, elimination of wet spots, and essential bridge repairs. This was completed on 24 March 2013, and new timetable changes have resulted in a morning train reaching Derry before 9am for the first time since Northern Ireland Railways took control of the network in the 1960s.

Phase 2 has seen the passing loop removed and the 'down' track lifted at Castlerock, replaced with a new loop further down the line at Bellarena halt. New signalling has been introduced, and the signal boxes at Castlerock and Waterside, Derry closed, with the line operating under absolute block. An hourly service between Belfast and Derry was introduced on 3 July 2017. [1] ^[7]

Phase 3 will include rail renewal between Castlerock and Eglinton, the introduction of a 90 mph (140 km/h) line speed between Castlerock and Derry and other works, however funding for this part of the project is doubtful for the foreseeable future.

Other future plans for the Derry line include the reinstatement of the double line from Antrim to Ballymena, the doubling of the track from Monkstown to Templepatrick, and relocation of the route terminus in Derry.

Coleraine has a bascule bridge for the railway over the navigable River Bann.^[4] Just after Castlerock station there are two tunnels created during an event known as the Great Blast in October 1845. Castlerock tunnel is 668 yards (611 m) long and is the longest operational railway tunnel in Northern Ireland. After passing through a short opening trains pass through the shorter Downhill tunnel which is 301 yards (275 m) in length.^[5]

Signalling on the line from Great Victoria Street to Slaught level crossing (just south of Ballymena station) is controlled by the Belfast Central control terminal. From Kellswater South, the signalling and level crossings are controlled by the Coleraine signal cabin. Following the signalling upgrade in 2016, the line north of Coleraine is no longer controlled by electric token, instead being centralised in the Coleraine signal box. The signal boxes at Castlerock and Waterside, which previously controlled the token system, have now been closed. The whole of the Belfast to Derry line is now controlled by colour light signals, the last semaphore signals at Castlerock station being removed after the 2016 signalling upgrade. The signal box at Coleraine will be moved to Lanyon Place upon completion of the Belfast Hub project.

Following the complete withdrawal of the NIR Class 80 and NIR Class 450 trains, the line is now served by a combination of NIR Class 3000 and NIR Class 4000 diesel multiple units.