South African Class MB 2-6-6-0
|South African Class MB 2-6-6-0
ex NGR Mallet 2-6-6-0 1910
NGR Mallet no. 337, later SAR Class MB no. 1602, circa 1910
|Type and origin|
|Designer||American Locomotive Company|
|Builder||American Locomotive Company|
|Serial number||48337-48341 |
|Gauge||3 ft 6 in (1,067 mm) Cape gauge|
|28 1⁄2 in (724 mm)|
|Driver diameter||45 1⁄2 in (1,160 mm)|
|Wheelbase||Total: 60 ft 9 1⁄4 in (18.523 m)
8 ft 4 in (2.540 m) per coupled set
33 ft 2 in (10.109 m) total
5 ft 10 in (1.778 m) bogie
17 ft 10 in (5.436 m) total
|Length||69 ft 1 in (21.057 m)|
|Height||12 ft 5 3⁄16 in (3.789 m)|
|Axle load||16.7 long tons (17.0 t) on 6th driver|
|Weight on drivers||82.2 long tons (83.5 t)|
|Locomotive weight||89.95 long tons (91.4 t)|
|Tender weight||45 long tons (45.7 t)|
|Locomotive and tender
|134.95 long tons (137.1 t)|
|Tender type||2 axle bogie
30 in (762 mm) wheels
|Fuel capacity||10 long tons (10.2 t)|
|Water capacity||4,000 imp gal (18,000 l)|
|Boiler||5 ft 8 in (1.727 m) inside diameter
16 ft 7.5 in (5.067 m) inside length
7 ft 6 in (2.286 m) pitch
|Boiler pressure||200 psi (1,380 kPa)|
|Firegrate area||45.5 sq ft (4.227 m2)|
|260 tubes 2 1⁄4 in (57.1 mm) diameter
2,546.2 sq ft (236.550 m2)
|– Firebox||154.6 sq ft (14.363 m2)|
|– Total||2,700.8 sq ft (250.913 m2)|
|17 1⁄2 in (444 mm) bore
26 in (660 mm) stroke
|28 in (711 mm) bore
26 in (660 mm) stroke
|Tractive effort||44,810 lbf (199 kN) at 50% pressure|
|Operator(s)||Natal Government Railways
South African Railways 
SAR Class MB
|Number in class||5|
SAR 1602-1606 
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.
Following on the satisfactory performance of the experimental Mallet compound steam locomotive that had been acquired by the Natal Government Railways (NGR) in 1909, a further five similar locomotives were ordered from the American Locomotive Company (ALCO) in 1910. They were delivered later that same year and numbered in the range from 337 to 341.
Like the previous Mallet locomotive, these five had Walschaerts valve gear, bar frames and used saturated steam. They differed little from the previous Mallet, basically only in respect of their larger boilers, which made them slightly heavier, and their tenders with a larger coal capacity.
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.
In the compound Mallet locomotive, 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. 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.
The five locomotives joined the first Mallet in banking service, working heavy coal trains between Estcourt and Highlands on the Natal mainline. To relieve coupler strain, they were used in three-locomotive trains and in this manner were able to haul 750 long tons (762.0 tonnes) up the one in thirty gradients on that line.
The Union of South Africa was established on 31 May 1910, in terms of the South Africa Act, enacted by the Parliament of the United Kingdom. One of the clauses in the Act required that the three Colonial Government railways, the Cape Government Railways, the NGR and the Central South African Railways, also be united under one single administration to control and administer the railways, ports and harbours of the Union. While the South African Railways (SAR) came into existence in 1910, the actual classification and renumbering of all the rolling stock of the three constituent railways required careful planning and was only implemented with effect from 1 January 1912.
- Articulated locomotive numbering and classification
- List of South African locomotive classes
- Mallet locomotive
- South African Class MA 2-6-6-0
- South African Class MC 2-6-6-0
- South African Class MC1 2-6-6-0
- South African Class MJ 2-6-6-0
- South African Class MJ1 2-6-6-0
- South African locomotive history
- The 2-6-6-0 Mallet
- Holland, D.F. (1972). Steam Locomotives of the South African Railways, Volume 2: 1910-1955 (1st ed.). Newton Abbott, Devon: David & Charles. pp. 13–14. ISBN 978-0-7153-5427-8.
- Classification of S.A.R. Engines with Renumbering Lists, issued by the Chief Mechanical Engineer’s Office, Pretoria, January 1912, pp. 9, 12, 15, 46 (Reprinted in April 1987 by SATS Museum, R.3125-6/9/11-1000)
- Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. pp. 84–85. ISBN 0869772112.
- Compounding Steam Engines
- The South African Railways - Historical Survey. Editor George Hart, Publisher Bill Hart, Sponsored by Dorbyl Ltd., Published c. 1978, p. 25.