South African Class MB 2-6-6-0

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NGR Mallet 2-6-6-0 1910
South African Class MB 2-6-6-0
SAR Class MB 1602 (2-6-6-0) NGR 337.jpg
NGR Mallet no. 337, SAR Class MB no. 1602, c. 1910
Type and origin
Power type Steam
Designer American Locomotive Company
Builder American Locomotive Company
Serial number 48337-48341
Model NGR Mallet
Build date 1910
Total produced 5
Configuration 2-6-6-0 (Denver)
Driver 3rd & 6th coupled axles
Gauge 3 ft 6 in (1,067 mm) Cape gauge
Leading dia. 28 12 in (724 mm)
Coupled dia. 45 12 in (1,156 mm)
Tender wheels 30 in (762 mm)
Wheelbase 60 ft 9 14 in (18,523 mm)
 • Engine 33 ft 2 in (10,109 mm)
 • Coupled 8 ft 4 in (2,540 mm) per unit
 • Tender 17 ft 10 in (5,436 mm)
 • Tender bogie 5 ft 10 in (1,778 mm)
 • Over couplers 69 ft 1 in (21,057 mm)
Height 12 ft 5 316 in (3,789 mm)
Frame type Bar
Axle load 16 LT 14 cwt (16,970 kg)
 • Leading 7 LT 15 cwt (7,874 kg)
 • 1st coupled 15 LT 5 cwt (15,490 kg)
 • 2nd coupled 14 LT 10 cwt (14,730 kg)
 • 3rd coupled 12 LT 9 cwt (12,650 kg)
 • 4th coupled 11 LT 10 cwt (11,680 kg)
 • 5th coupled 11 LT 16 cwt (11,990 kg)
 • 6th coupled 16 LT 14 cwt (16,970 kg)
 • Tender axle 11 LT 5 cwt (11,430 kg) average
Adhesive weight 82 LT 4 cwt (83,520 kg)
Loco weight 89 LT 19 cwt (91,390 kg)
Tender weight 45 LT (45,720 kg)
Total weight 134 LT 19 cwt (137,100 kg)
Tender type 2-axle bogies
Fuel type Coal
Fuel capacity 10 LT (10.2 t)
Water cap 4,000 imp gal (18,200 l)
Firebox type Round-top
 • Firegrate area 42.5 sq ft (3.95 m2)
 • Pitch 7 ft 6 in (2,286 mm)
 • Diameter 5 ft 8 in (1,727 mm)
 • Tube plates 16 ft 7 12 in (5,067 mm)
 • Small tubes 260: 2 14 in (57 mm)
Boiler pressure 200 psi (1,379 kPa)
Safety valve Ramsbottom
Heating surface 2,700.8 sq ft (250.91 m2)
 • Tubes 2,546.2 sq ft (236.55 m2)
 • Firebox 154.6 sq ft (14.36 m2)
Cylinders Four
High-pressure cylinder 17 12 in (444 mm) bore
26 in (660 mm) stroke
Low-pressure cylinder 28 in (711 mm) bore
26 in (660 mm) stroke
Valve gear Walschaerts
Couplers Johnston link-and-pin
Performance figures
Tractive effort 44,810 lbf (199 kN) @ 50%
Operators Natal Government Railways
South African Railways
Class NGR Mallet
SAR Class MB
Number in class 5
Numbers NGR 337-341
SAR 1602-1606
Delivered 1910
First run 1910
Withdrawn 1924

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

In November 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 designated Class MB.[1][2][3][4]


Following on the satisfactory performance of the experimental Mallet compound steam locomotive which 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 in November of that year and numbered in the range from 337 to 341.[1][2][4]


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 1 long ton 10 hundredweight (1.5 tonnes) larger coal capacity.[1][2][4]

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.[5]

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 which 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][4]


These 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 tonnes) up the one in thirty (3⅓%) gradients on that line.[1][4]

South African Railways[edit]

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

When the Union of South Africa was established on 31 May 1910, the three Colonial government railways (Cape Government Railways, NGR and Central South African Railways) were united under a single administration to control and administer the railways, ports and harbours of the Union.[6]

Although the South African Railways and Harbours came into existence in 1910, with Sir William Hoy appointed as its first General Manager, the actual classification and renumbering of all the rolling stock of the three constituent railways were only implemented with effect from 1 January 1912.[3][6]

In 1912, these five locomotives were renumbered in the range from 1602 to 1606 and designated Class MB on the South African Railways. They had a relatively short service life and were withdrawn from service by 1924. Their early demise was probably brought forward considerably by the appearance of the Garratt locomotive type on Cape Gauge in 1921.[1][3][4]


  1. ^ a b c d e f 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 Espitalier, T.J.; Day, W.A.J. (1945). The Locomotive in South Africa - A Brief History of Railway Development. Chapter VII - South African Railways. South African Railways and Harbours Magazine, April 1945. p. 274.
  3. ^ a b c 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)
  4. ^ 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. 
  5. ^ Compounding Steam Engines
  6. ^ a b The South African Railways - Historical Survey. Editor George Hart, Publisher Bill Hart, Sponsored by Dorbyl Ltd., Published c. 1978, p. 25.