 Some tank locomotive examples were also built, for which various suffixes to indicate the type of tank would be added to the wheel arrangement, for example for an engine with side-tanks.The 2-6-6-0 wheel arrangement was most often used for articulated compound steam Mallet locomotives.In a compound Mallet, the rear set of coupled wheels are driven by the smaller high pressure cylinders, from which spent steam is then fed to the larger low pressure cylinders that drive the front set of coupled wheels.