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 following a December 1900 editorial in "American Engineer and Railroad Journal". The notation counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, groups of numbers being separated by dashes. Other classification schemes, like UIC classification and the French, Turkish and Swiss systems for steam locomotives, count axles rather than wheels.
0-6-4
Under the Whyte notation for the classification of steam locomotives, represents the wheel arrangement of no leading wheels, six powered and coupled driving wheels on three axles, and four trailing wheels on two axles.
The 0-6-4 wheel arrangement appears to have only been used on tank engines and Single Fairlies. The earliest known example was the Moel Tryfan narrow gauge locomotive, built for use on the North Wales Narrow Gauge Railways. It was a Single Fairlie type, built by the Vulcan Foundry near Manchester in 1875. It was followed by the R class and S class, built by the Avonside Engine Company of England for the New Zealand Railways Department between 1878 and 1881.