From TrainSpottingWorld, for Rail fans everywhere

In Whyte notation, a 4-6-6-2 is a steam locomotive with four leading wheels (two axles) in an unpowered bogie at the front of the locomotive followed by two sets of driving wheels with six wheels each (three axles each), followed by two unpowered trailing wheels (one axle) at the rear of the locomotive.

This locomotive wheel arrangement was used in only a very limited number of locomotives in North America, most notably as class MM-2 cab forward locomotives on the Southern Pacific Railroad. These were effectively 2-6-6-4s running in reverse. They were originally built as 2-6-6-2s but were refitted with a four-wheel leading truck to increase stability at speed.

Cab forward steam locomotives of Southern Pacific Railroad
2-6-6-2: MM-2 (rebuilt as 4-6-6-2)
2-8-8-2: MC-1 · MC-2 · AC-1 · MC-4 · AC-2 · MC-6 · AC-3
4-8-8-2: AC-4 · AC-5 · AC-6 · AC-7 · AC-8 · AC-10 · AC-11 · AC-12
Preserved: SP 4294 (AC-12)