Under the Whyte notation for the classification of steam locomotives, a 4-8-4 locomotive has four leading wheels, eight coupled driving wheels and four trailing wheels.The equivalent UIC classification is 2'D2'.The 4-8-4 was an obvious progression from the 4-8-2 "Mountain" and the 4-6-4 "Hudson" types, combining the 4-8-2's ability to have more weight on the driving wheels (leading to greater traction, and allowing a larger, more powerful locomotive) and the 4-6-4's larger firebox supported by a 4-wheel trailing truck, allowing for freer steaming, particularly at speed.
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