Belfast–Derry line

The Belfast–Derry line (referred to as the Derry~Londonderry Line by NI Railways ) is an intercity railway line, running from Belfast to Derry in Northern Ireland.

The line is double-track on the short section it shares with the Belfast–Larne 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.

The line is the busiest single track railway line in the United Kingdom, carrying over 4 million passengers per annum, the Derry-Londonderry Line has also been described by Michael Palin as "one of the most beautiful rail journeys in the world".

Current service
Weekday and Saturday services on the line run hourly from in Belfast to  and vice versa, with some late-night and peak-time services starting from and terminating at. Hourly shuttle services to and from connect with the line at.

On Sunday, the service operates hourly from, however the outbound terminus alternates every hour between and  – with the effect that stations beyond  have a two-hourly service.

Upon the opening of Grand Central station later in 2024, services toward Belfast will terminate at that station instead.

Previous operations
Between 1978 and 2001, the line between the Bleach Green viaduct and Antrim was closed so 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.

Upgrade and Future
In 2011, it was planned to reduce services on the Coleraine to Derry~Londonderry section to five services, in each direction on weekdays, to facilitate safety improvement works during refurbishment of the line due to commence in 2012, but the £75 million that it was to cost was unavailable. This led to fears that the line would be permanently closed. Regional Development minister Danny Kennedy relocated funding from the A5 dualling project to the railway upgrade project, allowing for a 3-phase upgrade.

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

Phase 2 saw 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 was 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 in 2017.

Phase 3 will include rail renewal between Castlerock and Eglinton, the introduction of a 90 mph 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, and the doubling of the track from Monkstown to Templepatrick. The route terminus in Derry was relocated in 2019 when the former Belfast and Northern Counties Railway Waterside station reopened, replacing the 1980 terminus.

Railway engineering feature
Coleraine has a bascule bridge for the railway over the navigable River Bann. Just after Castlerock station there are two tunnels created during an event known as the Great Blast in October 1845. Castlerock tunnel is 668 yd 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 yd in length.

Signalling
Signalling on the line from Great Victoria Street to Slaught level crossing (just south of Ballymena station) is controlled by the Belfast Lanyon Place (Formally 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 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.

Rolling stock
Due to capacity issues on the line, Northern Ireland Railways announced they would be purchasing 21 extra carriages from Spanish manufacturer CAF. These units entered service in 2021. The line is typically served by 6 car Class 4000 sets, however, 3 car Class 3000s and Class 4000s sometimes operate on the line.