4-8-2+2-8-4

4-8-2+2-8-4 (Double Mountain)

KUR EC class, the first Double Mountain Garratt, 1926

Equivalent classifications UIC class 2D1+1D2 French class 241+142 Turkish class 47+47 Swiss class 4/7+4/7, 8/14 from the 1920s Russian class 2-4-1+1-4-2

First known tank engine version First use 1926 Country Kenya & Uganda Locomotive KUR EC class Railway Kenya-Uganda Railway Designer Beyer, Peacock and Company Builder Beyer, Peacock and Company

First known tender engine version First use 1938 Country South Africa Locomotive SAR Class GM Railway South African Railways Designer W.A.J. Day Builder Beyer, Peacock and Company Evolved from 2-8-2+2-8-2

Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a 4-8-2+2-8-4 is a Garratt articulated locomotive consisting of a pair of 4-8-2 engine units back to back, with the boiler and cab suspended between them. The 4-8-2 wheel arrangement has four leading wheels on two axles, usually in a leading bogie, eight powered and coupled driving wheels on four axles and two trailing wheels on one axle, usually in a trailing truck. Since the 4-8-2 type is generally known as a Mountain, the corresponding Garratt type is usually known as a Double Mountain.

A Garratt locomotive is actually two separate engines combined in a double articulated format, thereby providing multiple powered axles over which the total locomotive weight is spread. This results in a more powerful locomotive since, compared to a tender locomotive of similar total mass with tender included, a much larger percentage of the locomotive’s total mass contributes to traction.

The 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 would be to reduce the axle loading.

The largest steam locomotive built in Europe was a 4-8-2+2-8-4 Garratt, built by Beyer, Peacock and Company for the Soviet Railways in 1932. The most numerous Garratt class in the world was also a Double Mountain, the Class GMA and GMAM of the South African Railways, of which 120 were built between 1954 and 1958.