The Londonderry and Lough Swilly was the second-longest narrow-gauge rail network in Ireland. It served the north of County Donegal and its headquarters were in Derry. Its origins began in a decision to link Derry with Farland Point at the south-western corner of the Inishowen Peninsula. From there, steamers served the towns and villages on either shore of Lough Swilly - a very substantial inlet stretching from the Atlantic at Malin Head southwards to the 'capital' of Donegal, Letterkenny. The line was built to the Irish standard gauge of five feet three inches and, even as construction was underway, a branch was agreed up the eastern shore of Lough Swilly to the busy seaside town of Buncrana. In the event the Buncrana 'branch' proved to be the busiest part of the route and that to Farland Point was soon abandoned, the steamers transferring to Fahan pier a short distance south of Buncrana.

The line's success was largely due to Buncrana quickly becoming a favourite for day trippers from Derry but the steamer connections also meant that the railway formed part of a much wider-ranging transport network linking up much of north-east Donegal. That success did not go unnoticed and plans were quickly advanced for a line from Letterkenny to link up with the Buncrana line at the southern end of the Inishowen peninsula. This scheme, eventually combined with one to recover the saltmarsh 'sloblands' near Burnfoot, eventually took twenty years to come to fruition. Partly this was due to competing routes and partly because the new line was to be narrow gauge. The junction or more accurately interchange was made at the remote Tooban Junction. This comprised an island platform and signal box but no contact with the public roadway apart from a small footbridge for railway personnel only over an intervening stream. It was designed for passengers changing from one train to another, not for local access. Indeed, there were precious few locals to take advantage of the facility as it was located on the very edge of the reclaimed sloblands. Thus, for a few years, County Donegal had its very own 'break of gauge' causing the transfer of passengers and goods between trains.

Amalgamation between the Lough Swilly and Letterkenny railways followed shortly and with it the regauging of the former to provide a unified narrow-gauge network. Both routes were heavily used, especially for freight traffic. As with the Donegal Railway to the south, the Londonderry and Lough Swilly benefitted from government grants designed to improved the infrastructure of regions of the county regarded as 'overpopulated' or 'congested'. The first result was the extension of the Buncrana line further northwards around the Inishowen peninsula to eventually reach the market town of Carndonagh.

However, the most significant development was the establishment of the Letterkenny and Burtonport Extension Railway which, beginning with an end-on connection with the L&LSR at Letterkenny swung first southwards before turning northwards and, following the lie of the land, tracing a route towards Creeslough, near the north coast. Its route included passage of the Barnes Gap and the long Owencarrow Viaduct, subsequently the scene of the line's worst accident when a train was, literally, blown off the line by an Atlantic gale. From Creeslough, the L&BER turned westwards inland from the coast serving Falcarragh and then going south-west through the mountains and high turf bogs to Gweedore. The final section ran almost due west from remote Gweedore until, eventually, it reached the tiny fishing settlement of Burtonport.

The Swilly operated steam haulage throughout its life. Although the majority of the locos were tank engines of various sizes - including two massive 4-8-4T locos for the heavy freight workings and runs out to Burtonport, the line also operated two 4-8-0 tender locos, the only ones of this type to operate on an Irish narrow-gauge railway. These were mainly used on the Burtonport line. Traffic on the Letterkenny and Buncrana lines was always heavy and, at the former place, a connection was made with the County Donegal Railway's network.

In Derry, the company's headquarters were at Pennyburn in the north of the city and, after making a lengthy diagonal crossing of the main road, the line ran parallel to the quayside and the Harbour Commissioners' lines. The docks lines were partly dual gauge, 5' 3" and 3'. The L&LSR city terminus was at Graving Dock, a very undignified shed which did nothing to enhance the railway's prestige. Connection with the CDR's Victoria Road terminus on the other side of the River Foyle was made by the dock lines which ran on past Graving dock to the west end of the double-deck Craigavon bridge where a dual-gauge wagon turntable gave access to the lower deck's running line. A similar arrangement at the east end allowed wagons to be transferred to and from the CDR, although such transfers were rare. Derry, in fact, had four railways stations: L&LSR Graving Dock, CDR Victoria Road, GNR(I) Foyle Road, and B&NCR Waterside.

The partition of Ireland affected the L&LSR as dramatically as the CDR. Customs inspections caused considerable delays and effectively isolated County Donegal from its natural focus on Derry. This, combined with growing bus and lorry competition and rising repair and replacement costs seriously affected the L&LSR. The Carndonagh line was an early casualty and the line out to Burtonport was virtually abandoned beyond Gweedore soon after. The Second World War gave the Burtonport line a repreive however as turf fuel from the high bogs around Gweedore became a vital resource and kept the line going until 1947. Thw writing was on the wall, however, and the entire system closed in 1953.

That was not the end of the story. Although the company abandoned its trains it had been buying up competing bus and lorry services as well as adding its own. When the Swilly's trains stopped its buses and freight vehicles began to serve its passengers and customers. And, unique amongst all of Ireland's narrow-gauge railways, the Swilly continues to do so, albeit on roads rather than rails. For example, it is still possible to catch a Swilly bus at Derry's Foyleside bus terminus (built on the trackbed of the GNR(I) station) to Letterkenny and there change to the bus for Burtonport, following fairly closely the route taken by the railway.

The best book on the railway is that by E M Patterson, The Lough Swilly Railway, first published in 1964 and reprinted in 1988, David and Charles (Newton Abbot).