Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a 2-6-6-2 is a locomotive with one pair of unpowered leading wheels, followed by two sets of three pairs of powered driving wheels, and one pair of trailing wheels. The wheel arrangement was principally used on Mallet-type articulated locomotives, although some tank locomotive examples were also built. A Garratt type locomotive with the same wheel arrangement is classified as 2-6-0+0-6-2.
Other equivalent classifications are:
- UIC classification: 1CC1 (also known as German and Italian classifications)
- French classification: 130+031
- Turkish classification: 34+34
- Swiss classification: 3/4+3/4
- Russian classification: 1-3-0+0-3-1
Under the UIC classification the wheel arrangement is referred to as (1'C)C1' for Mallet locomotives.
The 2-6-6-2 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.
This type of locomotive was commonly used in North America on logging railroads. The 2-6-6-2 wheel arrangement was also used in the Soviet Union and in South Africa.
The Serbian Government used a Mallet articulated compound locomotive for freight service on 2 ft 6 in (762 mm) narrow gauge. It was built for the Serbian Government by the American Locomotive Company (ALCO).
The South African Railways (SAR) operated twenty-two Mallet locomotives with this wheel arrangement, spread over five classes, all of them built to 3 ft 6 in (1,067 mm) Cape gauge.
- In March 1910 the Central South African Railways (CSAR) placed a single experimental Mallet articulated compound steam locomotive in service. Ordered from ALCO, it was the first Mallet on the CSAR and, with its full working order weight of 157 long tons (159.5 tonnes; 175.8 short tons), it was the heaviest locomotive in the world working on 3 ft 6 in gauge at the time. It had Walschaerts valve gear and used saturated steam. In 1912, when it was assimilated into the SAR, it was classified as Class MD.
- In 1911 the CSAR placed nine compound Mallets in service. Also built by ALCO and with Walschaerts valve gear, they were very similar to the experimental Class MD, but they were equipped with Schmidt superheaters. In 1912, when they were assimilated into the SAR, they were classified as Class MF. Five more that were delivered in November 1911 were taken directly onto the SAR roster. In 1923 and 1925 six of them were converted to simple expansion (simplex) locomotives.
- Class MG.
- During 1911 the CSAR ordered an experimental simple expansion Mallet from the North British Locomotive Company (NBL). Compared to other South African Mallets this locomotive was unique, being arranged as a simplex locomotive with four high pressure cylinders instead of the more usual compound expansion arrangement. The locomotive was intended for test purposes on branch lines with light 45 pounds per yard (22 kilograms per metre) rail. It had Walschaerts valve gear, a plate frame and was equipped with a Schmidt superheater. By the time it was delivered in January 1912, the CSAR had already become part of the newly established SAR, who classified it as the sole Class ME
- In 1915 the SAR placed five Class MH compound Mallets in service, designed in detail in the locomotive drawing office in Pretoria under the direction of D.A. Hendrie, Chief Mechanical Engineer (CME) of the SAR from 1910 to 1922. They were superheated and had Walschaerts valve gear, and were ordered from NBL and erected in the Salvokop shops in Pretoria. At the time of their introduction, the Class MH was the largest and most powerful locomotive in the world on Cape gauge, with a full working order weight of 179.6 long tons (182.5 tonnes; 201.2 short tons).
The wheel arrangement also appeared in Soviet Russia as a 5 ft (1,524 mm) locomotive, the P34, built by Kolomna Locomotive Works. It was a modern but compact Mallet of which only one was built.
- 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. 54–56, 103–105, 138–140. ISBN 978-0-7153-5382-0.
- Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. pp. 86–87. 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. 16–19, 30–32, 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, 15-16, 46-47 (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