0-6-6-0

First known tank engine version
First known tank engine version
First use	1916
Country	Germany
Locomotive	Saxon Class XV HTV
Railway	Royal Saxon State Railways
Designer	Heinrich Lindner
Builder	Sächsische Maschinenfabrik

Equivalent classifications
Equivalent classifications
UIC class	(C)C
French class	030+030
Turkish class	33+33
Swiss class	3/3+3/3
Russian class	0-3-0+0-3-0

0-6-6-0
	Baltimore and Ohio’s Old Maude of 1904, the first 0-6-6-0 Mallet in the United States
UIC class
Equivalent classifications
UIC class	(C)C
French class	030+030
Turkish class	33+33
Swiss class	3/3+3/3
Russian class	0-3-0+0-3-0
First use
First known tank engine version
First use	1916
Country	Germany
Locomotive	Saxon Class XV HTV
Railway	Royal Saxon State Railways
Designer	Heinrich Lindner
Builder	Sächsische Maschinenfabrik
First use
First known tender engine version
First use	1904
Country	United States of America
Locomotive	Class O Old Maude
Railway	Baltimore and Ohio Railroad

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 and in some occasions, Double Fairlie locomotives.

A similar wheel arrangement exists for Double Fairlie, Meyer, Kitson-Meyer and Garratt locomotives, but on these types it is referred to as 0-6-0+0-6-0 since both engine units are pivoting.^[5]^[6]^[7]

^ Sagle, Laurence W. (1964). B&O Power: Steam, Diesel and Electric Power of the Baltimore and Ohio Railroad, 1829-1964. Alvin F. Staufer. p. 168. ISBN 0-944513-06-9.
^ Bruce, Alfred W. (1952). The Steam Locomotive in America. New York: Crown (Bonanza Books). p. 314.
^ Sagle (1964). B&O Power... p. 168.
^ Bruce (1952). The Steam Locomotive in America. p. 314.
^ Espitalier, T.J.; Day, W.A.J. (1943). The Locomotive in South Africa - A Brief History of Railway Development. Chapter II - The Adoption of the 3 ft. 6 in. Gauge on the Cape Government Railways (Continued). South African Railways and Harbours Magazine, August 1943. pp. 592-594.
^ Holland, D.F. (1971). Steam Locomotives of the South African Railways. Vol. 1: 1859–1910 (1st ed.). Newton Abbott, England: David & Charles. pp. 25–27. ISBN 978-0-7153-5382-0.
^ Dulez, Jean A. (2012). Railways of Southern Africa 150 Years (Commemorating One Hundred and Fifty Years of Railways on the Sub-Continent – Complete Motive Power Classifications and Famous Trains – 1860–2011) (1st ed.). Garden View, Johannesburg, South Africa: Vidrail Productions. p. 21. ISBN 9 780620 512282.