4-6-2+2-6-4

4-6-2+2-6-4 (Double Pacific)

SAR Class GF, the first 4-6-2+2-6-4 Double Pacific

Equivalent classifications UIC class 2C1+1C2 French class 231+132 Turkish class 36+36 Swiss class 3/6+3/6, 6/12 from 1920s Russian class 2-3-1+1-3-2

First known tank engine version First use 1927 Country South Africa Locomotive Class GF Railway South African Railways Designer South African Railways Builder Hannoversche Maschinenbau AG Evolved from 2-6-2+2-6-2

Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a 4-6-2+2-6-4 is a Garratt or Union Garratt articulated locomotive using a pair of 4-6-2 engine units back to back, with the boiler and cab suspended between them. The 4-6-2 wheel arrangement of each engine unit has four leading wheels on two axles, usually in a leading bogie, six powered and coupled driving wheels on three axles, and two trailing wheels on one axle, usually in a trailing truck. Since the 4-6-2 type is known as a Pacific, the corresponding Garratt type is usually known as a Double Pacific.

The Double Pacific type was fairly common for Garratt locomotives, especially those intended for faster passenger service. The first of the type was the Class GF, built by Hanomag for the South African Railways in 1927. The first to be built by Beyer, Peacock and Company, the owner of the Garratt patent, was the G class for the New Zealand Railways Department in 1928. Beyer, Peacock also built the last Double Pacific in 1943, for the Nigerian Railways.

The South African Railways also operated a Double Pacific version of the Union Garratt articulated locomotive. The Union Garratt was a hybrid locomotive, partly Modified Fairlie and partly Garratt. The front end was of a typical Garratt arrangement, with a water tank mounted on the front engine unit’s frame, while the rear end was constructed in the Modified Fairlie fashion, with the coal bunker mounted on a rigid extension of the locomotive’s main frame and with the pivoting rear engine unit positioned beneath the coal bunker. It had an additional large underbelly water tank under the boiler. The main frame therefore carried the smokebox, boiler, firebox, cab, coal bunker, as well as the underbelly water tank. As a result, like the Modified Fairlies, the Union Garratts were prone to metal fatigue and cracking of the frames. In the case of the Union Garratts, this occurred particularly at the rear, brought about by the long frame overhang laden with the coal bunker.