In the Whyte notation for classifying steam locomotives by wheel arrangement, a 4-6-6-4 is a railroad steam locomotive that has four leading wheels followed by two sets of six coupled driving wheels and four trailing wheels. 4-6-6-4's are commonly known as Challengers.
Union Pacific Challenger No. 3985 is an example of a 4-6-6-4 locomotive.
The Whyte notation is a classification method for steam locomotives, and some internal combustion locomotives and electric locomotives, by wheel arrangement. It 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.
Whyte notation from a handbook for railroad industry workers published in 1906