Under the Whyte notation for the classification of steam locomotives, a 0-6-6-0 wheel arrangement refers to a locomotive with two pivoting engine units or bogies, each with six coupled driving wheels, mounted under a rigid locomotive frame without any leading wheels or trailing wheels. The wheel arrangement was mostly used on Fairlie and Mallet locomotive types.
Other equivalent classifications are:
- UIC classification: CC (also known as German and Italian classifications)
- French classification: 030+030
- Turkish classification: 33+33
- Swiss classification: 3/3+3/3
The 0-6-6-0 wheel arrangement was used on Fairlie and Mallet locomotives. In the case of the Fairlie it is effectively two 0-6-0 locomotives operating back to back, while in the case of the Mallet the engine units were mounted in tandem.
The only compound Mallets to operate in Canada were the R1 class 0-6-6-0 Vaughan design locomotives, with the cylinder ends of the engine units facing each other. The class was owned by the Canadian Pacific Railway and served on the Big Hill in British Columbia, which had a 4.1% grade. Five locomotives were built between 1909 and 1911. A sixth one was built, but it was a simple expansion Mallet with two sets of high-pressure cylinders. All the locomotives in this class were later converted to 2-10-0 types and were used as shunting and transfer engines in Montreal.
Córas Iompair Éireann no. CC1, generally known as the Turf Burner, was a prototype 0-6-6-0 articulated steam locomotive designed by Oliver Bulleid. The locomotive shared some of the characteristics of Bulleid's previous attempt to develop a modern steam locomotive, the Southern Railway's Leader class. No. CC1 had a relatively short career and was never used in front-line service.
In 1876 the Cape Government Railways placed a single experimental 0-6-6-0 Fairlie side-tank locomotive in service on the Cape Eastern system, working out of East London. Built by Avonside Engine Company, it was the first articulated locomotive to enter service in South Africa and also the first locomotive to be equipped with Walschaerts valve gear. After some shortcomings were brought to the attention of the locomotive builders, a second Fairlie that incorporated these improvements was delivered and placed in service in 1878.
The only example of this type of engine in the United Kingdom was the Leader. It was originally commissioned by the Southern Railway but it was completed by British Railways in 1949. It was effectively a Meyer locomotive since both sets of drivers were articulated, C'C' under the UIC classification.
United States of America
The first Mallet locomotive built in the United States was of this type, the Baltimore and Ohio Railroad Class O no. 2400. The Kansas City Southern used the type as freight engines, with pilots, and had the most of them with twelve locomotives. The 0-6-6-0 wheel arrangement was also used to a limited extent on logging railroads and in mountain terminals.
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