4-4-2 (locomotive)

4-4-2 (Atlantic)

Front of locomotive at left

Milwaukee Road class A2 no. 919, 1901

Equivalent classifications UIC class 2B1 French class 221 Turkish class 25 Swiss class 2/5 Russian class 2-2-1

First known tank engine version First use 1880 Country United Kingdom Locomotive LT&SR 1 Class Railway London, Tilbury and Southend Railway Designer William Adams Builder Sharp, Stewart & Co. & Nasmyth, Wilson & Co. Evolved from Tank version of 4-4-0

First known tender engine version First use 1888 Country United States of America Locomotive Experimental double-firebox Designer George Strong Builder Hinkley Locomotive Works Evolved from 2-4-2 Benefits More stable than 2-4-2 Wide & deep firebox

Under the Whyte notation for the classification of steam locomotives by wheel arrangement, 4-4-2 represents a configuration of four leading wheels on two axles, usually in a leading bogie with a single pivot point, four powered and coupled driving wheels on two axles, and two trailing wheels on one axle, usually in a trailing truck which supports part of the weight of the boiler and firebox and gives the class its main improvement over 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 working.

While the wheel arrangement and type name Atlantic would come to fame in the fast passenger service competition between railroads in the United States by mid-1895, the tank locomotive version of the 4-4-2 Atlantic type first made its appearance in the United Kingdom in 1880, when William Adams designed the 1 Class 4-4-2T of the London, Tilbury and Southend Railway (LT&SR).

The 4-4-2T is the tank locomotive equivalent of a 4-4-0 American type tender locomotive, but with the frame extended to allow for a fuel bunker behind the cab. This necessitated the addition of a trailing truck to support the additional weight at the rear end of the locomotive. As such, the tank version of the 4-4-2 wheel arrangement appeared earlier than the tender version.