4-8-4+4-8-4

4-8-4+4-8-4 (Double Northern)

KUR EC3 class, the first Double Northern Garratt, 1939

Equivalent classifications UIC class 2D2+2D2 French class 242+242 Turkish class 48+48 Swiss class 4/8+4/8, 8/16 from the 1920s Russian class 2-4-2+2-4-2

First known tank engine version First use 1939 Country Kenya & Uganda Locomotive KUR EC3 class Railway Kenya-Uganda Railway Designer Beyer, Peacock and Company Builder Beyer, Peacock and Company Evolved from 4-8-2+2-8-4

Under the Whyte notation for the classification of steam locomotives by wheel arrangement, the 4-8-4+4-8-4 is a Garratt articulated locomotive. The wheel arrangement is effectively two 4-8-4 locomotives operating back to back, with the boiler and cab suspended between the two engine units. Each engine unit has two pairs of leading wheels in a leading bogie, followed by four coupled pairs of driving wheels and two pairs of trailing wheels in a trailing bogie. Since the 4-8-4 type is sometimes known as a Northern, the corresponding Garratt type would be referred to as a Double Northern.

There were only two classes of 4-8-4+4-8-4 steam locomotives across the globe, all of which were constructed by Beyer, Peacock and Company, the owners of the Garratt patent.

The predecessor 4-8-2+2-8-4 Double Mountain was probably the optimal Garratt wheel arrangement, with the four-wheeled leading bogies and the two-wheeled trailing trucks on each engine unit ensuring stability at speed and with sixteen coupled wheels for traction. More coupled wheels would inhibit the locomotive on tight curves, while the only advantage of more non-coupled wheels, such as on the Double Northern, was to reduce the axle loading.

The second Double Northern locomotive class were the AD60 class Garratts of the Australian 4 ft 8 ¹⁄₂ in (1,435 mm) standard gauge New South Wales Government Railways, of which 47 were delivered in 1952. Of these, 42 were delivered fully assembled while the last five were delivered in pieces as spare parts. The locomotive weighed 260 imperial tons and was the largest locomotive in the Southern Hemisphere. The last of the AD60 class entered service in 1956 and the last one was withdrawn from service in 1973.