The South African Railways Class MH 2-6-6-2 of 1915 was an articulated Mallet-design steam locomotive. In 1915, the South African Railways placed five Class MH Mallet articulated compound steam locomotives with a

2-6-6-2 Under the Whyte notation for the classification of steam locomotives by wheel arrangement, a is a locomotive with one pair of unpowered leading wheels, followed by two sets of three pairs of powered driving wheels and one pair of trailing wheels. ...

wheel arrangement in coal hauling service.Espitalier, T.J.; Day, W.A.J. (1945). ''The Locomotive in South Africa – A Brief History of Railway Development. Chapter VII – South African Railways (Continued).'' South African Railways and Harbours Magazine, May 1945. pp. 350, 356.

Manufacturer

During 1914, the requirement for locomotives with a high tractive effort to cope with the increasing volume of coal traffic between Witbank and Germiston led to the introduction of a heavy Mallet compound superheated engine with a 2-6-6-2 wheel arrangement.

The Class MH Mallet articulated locomotive was designed in detail in the locomotive drawing office in Pretoria under the personal direction of D.A. Hendrie, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1910 to 1922. The draughtsman, specially detached for the work, was J.R. Boyer who was later to become the Chief Locomotive Draughtsman of the SAR. Five of these very large locomotives were ordered from

North British Locomotive Company The North British Locomotive Company (NBL, NB Loco or North British) was created in 1903 through the merger of three Glasgow locomotive manufacturing companies; Sharp, Stewart and Company (Atlas Works), Neilson, Reid and Company (Hyde Park Wor ...

and delivered in 1915, numbered in the range from 1661 to 1665. They were erected in the Salvokop shops in Pretoria and were placed in service in September 1915.North British Locomotive Company works list, compiled by Austrian locomotive historian Bernhard Schmeiser

Characteristics

The main bar frames, thick, were machined from a wide solid. The hind part of this frame was rigidly secured to the boiler through the high-pressure cylinder saddle castings and terminated just in front of the firebox outer throat plate. From this point rearwards, the frame was of the plate type and arranged to carry the spring gear and other fittings for the trailing

Bissel truck A Bissell or Bissel truck (also Bissel bogie or Pony truck) is a single-axle bogie which pivots towards the centre of a steam locomotive to enable it to negotiate curves more easily. Invented in 1857 by and usually then known as a ''pony truck'' ...

. The locomotives were superheated and had

Walschaerts valve gear The Walschaerts valve gear is a type of valve gear used to regulate the flow of steam to the pistons in steam locomotives, invented by Belgium, Belgian railway mechanical engineering, engineer Egide Walschaerts in 1844. The gear is sometimes name ...

, controlled by steam reversing gear. The cylinders and steam chests were formed in three separate castings. The high-pressure cylinders were arranged with piston valves, while the low-pressure cylinders were arranged with Richardson balanced type slide valves, arranged above the cylinders. The steam chest covers of the low-pressure cylinders were designed with inclined joint faces to facilitate the handling of the valve and refacing of the ports during servicing. As built, the boiler pressure was set to blow off at , which gave the engine a tractive effort of at 50% of boiler pressure. The setting was later reduced to , which reduced the tractive effort to at 50% of boiler pressure. At the time of their introduction, the Class MH was the largest and most powerful locomotive in the world on Cape gauge. It attracted the attention of locomotive engineers throughout the world as an outstanding achievement for locomotive power on gauge. Its adhesive weight and the SAR's ultra-conservative practice of reporting a Mallet's tractive effort at only 50% of boiler pressure resulted in a much lower than actual starting tractive effort of . The Class MH was almost certainly capable of exerting more than tractive effort at starting.Soul of A Railway, System 7, Western Transvaal, based in Johannesburg, Part 21: Witbank Line by Les Pivnic, Eugene Armer, Peter Stow and Peter Micenko. Captions 3–4.
(Accessed on 4 May 2017)

Service

They were initially placed in service on the coal line between

Witbank Witbank (), officially Emalahleni, is a city situated on the Highveld of Mpumalanga, South Africa, within the Emalahleni Local Municipality. The name Witbank is Afrikaans for "white ridge", and is named after a white sandstone outcrop where wago ...

and

Germiston Germiston, also known as kwaDukathole, is a small city in the East Rand region of Gauteng, South Africa, administratively forming part of the City of Ekurhuleni Metropolitan Municipality since the latter's establishment in 2000. It functions as ...

as intended 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 Vryheid ( zu, IVryheid) is a coal mining and cattle ranching town in northern KwaZulu-Natal, South Africa. Vryheid is the Afrikaans word for "freedom". History After Boer farmers, who lived in the Vryheid area, had helped King Dinuzulu defeat his ...

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.

Illustration

File:Class MH 2-6-6-2 no. 1661.jpg, Class MH 2-6-6-2 Mallet no. 1661, c. 1915

References

{{Steam locomotive tenders 2280 2280 2-6-6-2 locomotives (1C)C1 locomotives NBL locomotives Cape gauge railway locomotives Railway locomotives introduced in 1915 1915 in South Africa Scrapped locomotives