4.2"

Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 4-4-2 represents a configuration of a four-wheeled leading bogie, four powered and coupled driving wheels, and two trailing wheels supporting part of the weight of the boiler and firebox. This allows a larger firebox and boiler than the 4-4-0 configuration.This wheel arrangement is commonly known as the Atlantic type, although it is also sometimes called a Milwaukee or 4-4-2 Milwaukee, after the Milwaukee Road, which employed it in high speed passenger service.

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