In the 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...

 for describing 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...

 wheel arrangement

Wheel arrangement

In rail transport, a wheel arrangement is a system of classifying the way in which wheels are distributed beneath a locomotive.. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country...

, a 2-6-6 is a locomotive with a two-wheeled leading truck, six driving wheel

Driving wheel

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

s, and a six-wheeled trailing truck. All the locomotives produced of this arrangement have been tank locomotive

Tank locomotive

A tank locomotive or tank engine is a steam locomotive that carries its water in one or more on-board water tanks, instead of pulling it behind it in a tender. It will most likely also have some kind of bunker to hold the fuel. There are several different types of tank locomotive dependent upon...

s, and the vast majority in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. It was a popular arrangement for the larger Mason Bogie

Mason Bogie

The Mason Bogie is a type of articulated steam locomotive suited for sharp curves and uneven track, once commonly used on narrow gauge lines in the United States...

s, as well as some of the largest suburban tank locomotives.

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...
  
  : 1C3 (also known as German classification and Italian classification)
• French classification: 133
• 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...
  
  : 37
• Swiss classification: 3/7