In Whyte notation

Whyte notation

The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal...

, a 0-6-6-0 is a railroad steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

 that has two articulated sections, each with six coupled driving wheel

Driving wheel

On a steam locomotive, a driving wheel is a powered wheel which is driven by the locomotive's pistons...

s, without any leading wheel

Leading wheel

The leading wheel or leading axle of a steam locomotive is an unpowered wheel or axle located in front of the driving wheels. The axle or axles of the leading wheels are normally located in a truck...

s or trailing wheel

Trailing wheel

On a steam locomotive, a trailing wheel or trailing axle is generally an unpowered wheel or axle located behind the driving wheels. The axle of the trailing wheels was usually located on a trailing truck...

s.

Equivalent classifications

Other equivalent classifications are:

• UIC classification
  UIC classification
  The UIC classification of locomotive axle arrangements describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways "Leaflet 650 - Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much...
  
  : CC (also known as German classification and Italian classification)
• French classification: 030+030
• Turkish classification
  Turkish classification
  In the Turkish classification system for railway locomotives, the number of powered axles are followed by the total number of axles. It is identical to the Swiss system except that the latter places a slash between the two numbers.Thus0-6-0 becomes 33...
  
  : 33+33
• Swiss classification: 3/3+3/3

United States

The first Mallet locomotive

Mallet locomotive

The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet ....

 built in the United States, Baltimore and Ohio Railroad

Baltimore and Ohio Railroad

The Baltimore and Ohio Railroad was one of the oldest railroads in the United States and the first common carrier railroad. It came into being mostly because the city of Baltimore wanted to compete with the newly constructed Erie Canal and another canal being proposed by Pennsylvania, which...

 #2400 (Class O

Baltimore and Ohio Railroad locomotives

On the Baltimore and Ohio Railroad, locomotives were always considered of great importance, and the railroad was involved in many experiments and innovations.-Early locomotives:...

), was of this type. 0-6-6-0's were also used, in limited amounts, on logging railroads and in mountain terminals.

United Kingdom

The only example of this type of engine in the UK 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

Meyer locomotive

A Meyer locomotive is a type of articulated locomotive. The design was never as popular as the Garratt or Mallet locomotives. It can be best regarded as 19th Century competition for the early compound Mallet and also the Fairlie articulated designs....

 since both sets of drivers were articulated. In UIC classification this would be written C'C'.

Ireland

Córas Iompair Éireann

Córas Iompair Éireann

Córas Iompair Éireann , or CIÉ, is a statutory corporation of the Irish state, answerable to the Irish Government and responsible for most public transport in the Republic of Ireland and, jointly with its Northern Ireland counterpart, the Northern Ireland Transport Holding Company, between the...

 No. CC1, generally known as the Turf Burner

CIÉ No. CC1

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 to burn turf and built at CIÉ's Inchicore Works in Dublin. CC1 shared some, but not all, of the characteristics of Bulleid's previous attempt to...

, was a prototype 0-6-6-0 articulated steam locomotive designed by Oliver Bulleid

Oliver Bulleid

Oliver Vaughan Snell Bulleid was a British railway and mechanical engineer best known as the Chief Mechanical Engineer of the Southern Railway between 1937 and the 1948 nationalisation, developing many well-known locomotives.- Early life and Great Northern Railway :He was born in Invercargill,...

  CC1 shared some of the characteristics of Bulleid's previous attempt to develop a modern steam locomotive, the Leader

SR Leader Class

The Leader was a class of experimental 0-6-6-0T articulated steam locomotive, produced in the United Kingdom to the design of the innovative engineer Oliver Bulleid. The Leader was an attempt to extend the life of steam traction by eliminating many of the operational drawbacks associated with...

. CC1 had a relatively short career and was never used in front-line service.

Canada

The only compound mallets to operate in Canada were R1 class 0-6-6-0 Vaughan design (cylinders back to back)locomotives, owned by the Canadian Pacific Railway, for service on the Big Hill

Big Hill

The Big Hill on the Canadian Pacific Railway main line in British Columbia, Canada, was the most difficult piece of railway track on the Canadian Pacific Railway's route. It was situated in the rugged Canadian Rockies west of the Continental Divide and Kicking Horse Pass...

in British Columbia, which had a 4.1% grade. They were built between 1909 and 1911. A 6th one was built, but had 2 sets of high-pressure cylinders, and was not a compound mallet. All of the locomotives in this class were converted to 2-10-0 types, and were used as shunting and transfer engines in Montreal.