In the Whyte notation for describing steam locomotive wheel arrangement, a 2-6-6 is a locomotive with a two-wheeled leading truck, six driving wheels, and a six-wheeled trailing truck. All the locomotives produced of this arrangement have been tank locomotives, and the vast majority in the United States. It was a popular arrangement for the larger Mason Bogies, as well as some of the largest suburban tank locomotives.
Other equivalent classifications are:
- UIC classification: 1C3 (also known as German classification and Italian classification)
- French classification: 133
- Turkish classification: 37
- Swiss classification: 3/7