South African Class MB 2-6-6-0

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South African Class MB 2-6-6-0
ex NGR Mallet 2-6-6-0 1910
SAR Class MB 1602 (2-6-6-0) NGR 337.jpg
NGR Mallet no. 337, later SAR Class MB no. 1602, circa 1910
Type and origin
Power type Steam
Designer American Locomotive Company
Builder American Locomotive Company
Serial number 48337-48341 [1]
Model NGR Mallet
Build date 1910
Total produced 5
Configuration 2-6-6-0 Mallet
Gauge 3 ft 6 in (1,067 mm) Cape gauge
Leading wheel
28 12 in (724 mm)
Driver diameter 45 12 in (1,160 mm)
Wheelbase Total: 60 ft 9 14 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 5 316 in (3.789 m)
Frame Bar frame
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
combined weight
134.95 long tons (137.1 t)
Tender type 2 axle bogie
30 in (762 mm) wheels
Fuel type Coal
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)
Heating surface:
– Tubes
260 tubes 2 14 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)
Cylinders Four
cylinder size
17 12 in (444 mm) bore
26 in (660 mm) stroke
cylinder size
28 in (711 mm) bore
26 in (660 mm) stroke
Valve gear Walschaerts
Performance figures
Tractive effort 44,810 lbf (199 kN) at 50% pressure
Operator(s) Natal Government Railways
South African Railways [1]
Class NGR Mallet
SAR Class MB
Number in class 5
Number(s) NGR 337-341
SAR 1602-1606 [1][2]
Delivered 1910
First run 1910
Withdrawn 1924

The South African Class MB 2-6-6-0 of 1910 is a South African steam locomotive from the pre-Union era in the Natal Colony.

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.[1][2][3]


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.[1][3]


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.[1][3]

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.[4]

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.[1][3]


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.[1][3]

Railway network of the Natal Government Railways in 1910 upon the establishment of the Union of South Africa and the South African Railways

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.[2][5]

In 1912 these five locomotives were renumbered in the range from 1602 to 1606 and classified as Class MB on the SAR. They were withdrawn from service by 1924.[1][2][3]

See also[edit]


  1. ^ a b c d e f g h i 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. 
  2. ^ a b c d 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)
  3. ^ a b c d e f Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. pp. 84–85. ISBN 0869772112. 
  4. ^ Compounding Steam Engines
  5. ^ The South African Railways - Historical Survey. Editor George Hart, Publisher Bill Hart, Sponsored by Dorbyl Ltd., Published c. 1978, p. 25.