In 1915 the South African Railways placed five Class MH Mallet articulated compound steam locomotives

Compound locomotive

A compound engine unit is a type of steam engine where steam is expanded in two or more stages.A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger...

 with a 2-6-6-2

2-6-6-2

In Whyte notation, 2-6-6-2 refers to a railroad steam locomotive that has two leading wheels followed by six coupled driving wheels, a second set of six coupled driving wheels, and two trailing wheels...

 wheel arrangement in service.

Manufacturer

The Class MH 2-6-6-2 Mallet articulated compound steam locomotive was designed in detail in the locomotive drawing office in Pretoria under the direction of D.A. Hendrie, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1910 to 1922. Five of these very large locomotives were ordered from North British Locomotive Company

North British Locomotive Company

The North British Locomotive Company was created in 1903 through the merger of three Glasgow locomotive manufacturing companies; Sharp Stewart and Company , Neilson, Reid and Company and Dübs and Company , creating the largest locomotive manufacturing company in Europe.Its main factories were...

 (NBL) and delivered in 1915, numbered 1661 to 1665.

They were superheated and had Walschaerts valve gear. The locomotives were erected in the Koedoespoort shops in Pretoria and were placed in service in September 1915.

Compound expansion

In a compound locomotive steam is expanded in phases. After being expanded in a high pressure cylinder and having then lost pressure and given up part of its heat, it is exhausted into a larger volume low pressure cylinder for secondary expansion, after which it is exhausted through the smokebox. By comparison, in the more usual arrangement of simple expansion, steam is expanded just once in any one cylinder before being exhausted through the smokebox.

In the compound Mallet locomotive

Mallet locomotive

The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet ....

, the rear set of coupled wheels are driven by the smaller high pressure cylinders which are fed steam from the steam dome. Their spent steam is then fed to the larger low pressure cylinders that drive the front set of coupled wheels.

Characteristics

At the time of their introduction, the Class MH was the largest and most powerful locomotive in the world on Cape gauge

Cape gauge

Cape gauge is a track gauge of between the inside of the rail heads and is classified as narrow gauge. It has installations of around .The gauge was first used by Norwegian engineer Carl Abraham Pihl and the first line was opened in 1862.- Nomenclature :...

. It attracted the attention of locomotive engineers throughout the world as an outstanding achievement for locomotive power on 3 in 6 in (1.07 m) gauge.

Service

They were placed in service on the coal line between Witbank and Germiston that they were intended for, to supplement the other Mallets already working on that line. In the 1930s they were transferred to Natal to work on the line between Vryheid and Glencoe, also hauling coal. They were outstanding in their performance and remained in Natal for the rest of their service lives, until they were all retired and scrapped by 1940.

See also

• Mallet locomotive
  Mallet locomotive
  The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet ....
  
  
• South African Class MA 2-6-6-0
  South African Class MA 2-6-6-0
  In 1909 the Natal Government Railways placed a single 2-6-6-0 Mallet articulated compound steam locomotive in service. In 1912, when it was assimilated into the South African Railways, it was renumbered and classified as Class MA.-Manufacturer:...
  
  
• South African Class MB 2-6-6-0
  South African Class MB 2-6-6-0
  In 1910 the Natal Government Railways placed five 2-6-6-0 Mallet articulated compound steam locomotives in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and classified as Class MB.-Manufacturer:...
  
  
• South African Class MC 2-6-6-0
  South African Class MC 2-6-6-0
  In 1912 the South African Railways placed ten Class MC Mallet articulated compound steam locomotives with a 2-6-6-0 wheel arrangement in service.-Manufacturer:...
  
  
• South African Class MC1 2-6-6-0
  South African Class MC1 2-6-6-0
  In 1914 the South African Railways placed fifteen Class MC1 Mallet articulated compound steam locomotives with a 2-6-6-0 wheel arrangement in service.-Manufacturer:...
  
  
• South African Class MD 2-6-6-2
  South African Class MD 2-6-6-2
  In March 1910 the Central South African Railways placed a single experimental Mallet articulated compound steam locomotive with a 2-6-6-2 wheel arrangement in service...
  
  
• South African Class ME 2-6-6-2
  South African Class ME 2-6-6-2
  In January 1912 the South African Railways placed a single Class ME Mallet articulated steam locomotive with a 2-6-6-2 wheel arrangement in service.-Manufacturer:...
  
  
• South African Class MF 2-6-6-2
  South African Class MF 2-6-6-2
  In 1911 the Central South African Railways placed nine Mallet articulated compound steam locomotives with a 2-6-6-2 wheel arrangement in service. In 1912, when they were assimilated into the South African Railways, they were renumbered and classified as Class MF...
  
  
• South African Class MG 2-6-6-2
  South African Class MG 2-6-6-2
  In 1911 the Central South African Railways placed a single experimental Mallet articulated compound steam locomotive with a 2-6-6-2 wheel arrangement in service...
  
  
• South African Class MJ 2-6-6-0
  South African Class MJ 2-6-6-0
  Between 1914 and 1921 the South African Railways placed eighteen Class MJ Mallet articulated compound steam locomotives with a 2-6-6-0 wheel arrangement in branch line service.-Manufacturers:...
  
  
• South African Class MJ1 2-6-6-0
  South African Class MJ1 2-6-6-0
  In 1918 the South African Railways placed eight Class MJ1 Mallet articulated compound steam locomotives with a 2-6-6-0 wheel arrangement in branch line service.-Manufacturer:...
  
  
• Articulated locomotive numbering and classification
• The 2-6-6-2 Mallet
  2-6-6-2
  In Whyte notation, 2-6-6-2 refers to a railroad steam locomotive that has two leading wheels followed by six coupled driving wheels, a second set of six coupled driving wheels, and two trailing wheels...
  
  
• List of South African locomotive classes