Under the Whyte notation for the classification of steam locomotives, a 0-6-6-0 wheel arrangement refers to a locomotive with two engine units mounted under a rigid locomotive frame, with the front engine unit pivoting and each engine unit with six coupled driving wheels without any leading or trailing wheels. The wheel arrangement was mostly used to describe Mallet locomotive types.
A similar wheel arrangement exists for Fairlie, Meyer, Kitson-Meyer and Garratt articulated locomotives, but on these types it is referred to as 0-6-0+0-6-0 since both engine units are pivoting.
Other equivalent classifications are:
- UIC classification: C'C' (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 Mallet locomotives, on which the engine units were mounted either in tandem or facing each other.
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.
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.
The Western Maryland Railway had a small fleet of 2-6-6-2 locomotives which, at one time, were the heaviest locomotives in the world, weighing 264 Tons. They were all converted to 0-6-6-0 locomotives for heavy switching.
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