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.
They were originally built as 2-6-6-2s but were refitted with a four-wheel leading truck to increase stability at speed.
Southern Pacific AM-2s were built from July to August 1911 by Baldwin Locomotive Works as Cab Forwards.
These 4-6-6-2s began retirement in the mid- to late-1930s, although a few remained in operation until the end of World War II.
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