The Dalkey Atmospheric Railway (unofficial opening 19 August 1843, official opening 29 March 1844 – 12 April 1854) was an extension of the Dublin and Kingstown Railway to Atmospheric Road in Dalkey, Co. Dublin, Ireland. It used part of the Dalkey Quarry industrial tramway, which was earlier used for the construction of Kingstown (Dún Laoghaire) Harbour. It was the first railway of its type in the world.
The standard gauge line was 9,200 feet (2,800 m) in length with an average uphill gradient of about 1 in 110. Vacuum power via a 15-inch (380 mm) pipe was used for the ascent to Dalkey, speeds of up to 40 mph (64 km/h) being achieved, and the return journey was by means of gravity. The vacuum tube fell 560 yd (512 m) short of the Dalkey station, and the train relied on momentum for the last stretch of the journey. To commence the journey to Kingstown the train had to be pushed by hand until the piston engaged with the tube.
The success of the railway led to reports that plans were drawn up in 1843 to extend the line to Bray, however this did not come to fruition.
The line was visited by Isambard Kingdom Brunel and other representatives of the Great Western Railway which subsequently constructed the 20 mi (32 km) South Devon Railway and operated for a year with atmospheric propulsion.
In November 1843 the French Government sent Mons. Mallet to conduct a detailed study of the railway (including measurements made with Joseph Samuda). The extensive report was sufficiently favourable to lead to the construction of the 5.3 mi (8.5 km) Saint-Germain atmospheric railway near Paris, which was built in 1847 and operated until 1860.
Robert Stephenson also reported in 1844 to the Chester and Holyhead railway on an extensive series of tests he devised for the line, which were carried out on his behalf by Mr G Berkley, and Mr W.P. Marshal. Stephenson looked at the applicability of the atmospheric system to a variety of purposes, from inclines to main line, and concluded that it only had economic advantage compared to rope incline or locomotive hauled on short lines (e.g. 3 to 5 miles length) having light trains with frequent departures, especially where the gradients precluded the use of locomotives.
William Dargan was the contractor and Charles Vignoles the engineer. The atmospheric equipment was supplied by Samuel Clegg and Jacob and Joseph Samuda. The vacuum was provided by a 100H.P. single cylinder steam engine at Dalkey. This was a condensing engine with steam supplied at 40psi. The steam cylinder was 34.5 inches in diameter and the air pump 67 inches diameter, both with 5.5ft stroke and capable of 22 strokes per minute. This engine was claimed to be suitable for a 6-mile stretch of railway, and was therefore lightly loaded.
Trains ran every half-hour between 8:00am and 6:00pm. Some clue as to the size of the trains can be obtained from the study carried out from the French Government in 1843. A train of 38 tons gross weight was described which comprised 7 carriages and carried 200 people, and in subsequent tests train weights of up to 70 tons are reported. The journey to Dalkey could be completed in just over 3 minutes with the speed being limited by the need to brake for curves on the line, and speeds of over 40mph could be reached.
In 1854 the line was closed to allow for conversion to 1,600 mm (5 ft 3 in) (Irish gauge) and thus integration into the Dublin and South Eastern Railway.
A section of the path of the line now forms part of the DART route. The last 200 metres, or so, at the Dalkey end ran slightly to the north of the present line and is now derelict or built over. The bridge which carried Castle Park Road over the atmospheric railway is still in existence and everyday use. The pumping station was sited in the grounds of a house which still stands beside the path called 'The Metals' adjacent to Barnhill Road.