Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a 2-6-6-0 is a locomotive with one pair of unpowered leading wheels, followed by two sets of three pairs of powered driving wheels, and no trailing wheels. The wheel arrangement was principally used on Mallet-type articulated locomotives, although some tank locomotive examples were also built.
Other equivalent classifications are:
- UIC classification: 1CC (also known as German and Italian classifications)
- French classification: 130+030
- Turkish classification: 34+33
- Swiss classification: 3/4+3/3
The 2-6-6-0 wheel arrangement was most often used for articulated compound steam Mallet locomotives. In a compound Mallet, the rear set of coupled wheels are driven by the smaller high pressure cylinders, from which spent steam is then fed to the larger low pressure cylinders that drive the front set of coupled wheels.
The sole NZR E class locomotive of 1906 was the only 2-6-6-0 tank locomotive ever built for and used by the New Zealand Railways Department. It was built at the Petone Workshops in Wellington and was designed for use on the world famous Rimutaka Incline. Numbered 66, making it E 66, it spent the first part of its working life in the Wellington region hauling trains up and down the Rimutaka Incline. It was eventually transferred to the Wellington-Johnsonville section for banking duties, even though it was not designed for that type of work. In 1917 E 66 was withdrawn from service and scrapped. Sadly, it didn't survive long enough for preservation.
The South African Railways (SAR) operated fifty-seven Mallet locomotives with this wheel arrangement, spread over six classes, all of them 3 ft 6 in (1,067 mm).
- In 1909 the Natal Government Railways (NGR) placed a single experimental 2-6-6-0 Mallet articulated compound steam locomotive in service. Built by the American Locomotive Company (ALCO), this was the first Mallet type compound steam locomotive to enter service in South Africa. The locomotive had Walschaerts valve gear, a bar frame and used saturated steam. At the time it was the most powerful locomotive in the country. In 1912, when it was assimilated into the SAR, it was classified as Class MA.
- In 1910, following on the satisfactory performance of the experimental Mallet, the NGR placed five more in service, also built by ALCO. These five differed little from the previous Mallet, basically only in respect of larger boilers which made them slightly heavier and tenders with a larger coal capacity. In 1912, when they were assimilated into the SAR, they were classified as Class MB.
- In 1912 the SAR placed ten Class MC Mallets in service. Built by the North British Locomotive Company (NBL), these also had Walschaerts valve gear and used saturated steam. Although their tenders were similar to those of the SAR’s Class 3 4-8-2, they differed little from the previous Mallets in size, power and performance. Two of them were equipped with superheaters at a later date, but no others were modified in this manner.
- In 1914 the SAR placed fifteen Class MC1 Mallets in service. Ordered from NBL in 1913, they were an improved version of the Class MC with a redesigned boiler that included a superheater and with 0.5 inches (12.7 millimetres) larger diameter low pressure and high pressure cylinders. The result was a much better performing locomotive with an increased tractive effort.
- Between 1914 and 1921 the SAR placed eighteen Class MJ Mallets in branch line service. Designed by D.A. Hendrie, Chief Mechanical Engineer (CME) of the SAR from 1910 to 1922, they were superheated and had Walschaerts valve gear and Belpaire fireboxes. Ten were ordered from Maffei but, as a result of the outbreak of World War I, only two could be delivered from Germany in 1914. The order for the remaining eight was then transferred to NBL, who delivered them in 1917. After the cessation of hostilities, Maffei requested that the already built balance of the original order also be accepted. These eight locomotives were delivered in 1921.
- In 1918 the SAR placed eight Class MJ1 Mallets in branch line service. Because of the disruption of British and German locomotive builders during World War I, they were ordered Montreal Locomotive Works (MLW) in Canada. The locomotive design was by MLW, based on the specifications for the Class MJ Mallet. Like the Class MJ, they were superheated, with Walschaerts valve gear and Belpaire fireboxes, but with slightly larger boilers and with the sandbox mounted on the boiler in North American style. The Class MJ1 was the last Mallet type to be placed in service by the SAR and all its subsequent new articulated locomotives were to be Modified Fairlies and Garratts.
United States of America
The only American railroad to purchase 2-6-6-0 locomotives was the Denver, Northwestern and Pacific Railway, which later became the Denver and Salt Lake Railroad and eventually the Denver and Salt Lake Railway. Towards the end of their service life, after the acquisition of the Denver and Salt Lake, these locomotives were used by the Denver and Rio Grande Western Railroad (D&RGW).
- Compounding Steam Engines
- Holland, D.F. (1971). Steam Locomotives of the South African Railways, Volume 1: 1859-1910 (1st ed.). Newton Abbott, Devon: David & Charles. pp. 103–105. ISBN 978-0-7153-5382-0.
- Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. pp. 84–85, 87–88. ISBN 0869772112.
- Holland, D.F. (1972). Steam Locomotives of the South African Railways, Volume 2: 1910-1955 (1st ed.). Newton Abbott, Devon: David & Charles. pp. 13–14, 21–22, 29, 36–37, 140. ISBN 978-0-7153-5427-8.
- Classification of S.A.R. Engines with Renumbering Lists, issued by the Chief Mechanical Engineer’s Office, Pretoria, January 1912, pp. 9, 12, 15, 46 (Reprinted in April 1987 by SATS Museum, R.3125-6/9/11-1000)
- North British Locomotive Company works list, compiled by Austrian locomotive historian Bernhard Schmeiser
- South African Railways and Harbours Locomotive Diagram Book, 2’0” & 3’6” Gauge Steam Locomotives, 15 August 1941, as amended
- Hendrie (10 December 1921). "Engine Power on the S.A.R.". South African Mining and Engineering Journal. XXXII (1576): 529.
- North British Locomotive Co. works list (from J. Lambert)
- Durrant, A E (1989). Twilight of South African Steam (1st ed.). Newton Abbott, London: David & Charles. pp. 24–25. ISBN 0715386387.