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Technical Info - Home

verview Overview:
The first commercially successful steam engine was a stationary pumping engine built in 1698 by Thomas Savery (Fig. 1). Development was steady until James Watt in 1763 produced the independent condenser, and later a number of other devices, which made the stationary steam engine a really useful machine.
While earlier attempts to construct steam road carriages had been made, the first locomotive to run on rails was built in 1802 by Richard Trevithick in Cornwall, England. In 1804 he built an improved locomotive, which hauled coal wagons on the Merthyr Tydvil Railway in South Wales, which is believed to be the oldest steam worked railway in the world.
From then on development of the steam locomotive was very rapid, until in 1829 George Stephenson and his son Robert built the famous locomotive "The Rocket" (Fig. 2), which won a £500 prize offered by the Liverpool and Manchester Railway, in England.
The "Rocket" was fitted with a multi-tubular boiler, a water jacketed firebox, a smokebox, and had two outside cylinders coupled to a single pair of driving wheels.  The exhaust steam from the cylinders was lead to a blast pipe in the smokebox to produce the necessary draught. The basic principles of the steam locomotive as used today were thus laid down in 1829. Except for developments in detail and in size, no fundamental changes have been made since then.

In South Africa a short railway was opened in June 1860, from the Point Docks to Durban, and in May 1861, the first portion of the railway from Cape Town to Wellington was opened.  From then on railway development was very rapid throughout the country. T
he month of May 1910 saw the Cape, Natal and the Transvaal Government Railways, as well as the Ports of those provinces all combine to form South African Railways and Harbours (SAR&H). On the 1st April 1990, Transnet was formed, and the South African Railways became Spoornet, a major division of Transnet 

Rail Gauges:
The first gauge used in South Africa in 1859 was the 4' 8½" (Fig. 1). This was used on the privately owned Cape and Natal Railways long before the SAR, and conformed to the British standard. Apart from being very costly to lay, this was a very impractical gauge for the South African terrain, due to the sharp curves required to negotiate the landscape.
Another privately owned railway was also built to carry copper from inland to the coast in the north-western Cape. This line had a gauge of 2' 6" (Fig. 2), and was in existence at the same time as the Cape and Natal gauge of 4' 8½". Although this was a lot more suited to the landscape, problems arose when locomotives from each gauge could not be used on all lines. In 1873 a decision was made by the Cape Parliament to standardise on a single gauge, and a compromise was made between the 4' 8½" and 2' 6" gauges. It was then that the 3' 6" Cape gauge was born.


Although classed as Narrow Gauge when compared to world standards, the 3' 6" gauge (Fig. 3) has proved to be the more popular gauge suited to the South African environment. It is the only official gauge in use by Transnet today. It has a loading gauge of 13' 0" (Fig. 3), and is available in various weights for main and branch lines.

There were areas, particularly in the Cape and Natal, that were too rugged for the standard 3' 6" gauge to handle the sharp curves of the terrain. The Cape and Natal are well known for their mountainous and very hilly landscapes. It was for this reason that
2' 0" gauge lines (Fig.4) were constructed primarily in the Cape and Natal provinces. These lines were classed as Narrow Gauge (NG) in South Africa. Although these lines were a lot cheaper to construct, problems arose again, when having to join with the wider Cape gauge lines. There are only a few Narrow Gauge lines left in South Africa, and are being maintained by preservation societies.

Wheel Arrangement:

All locomotives have a specific number of wheels. These could be intermediate wheels, bogies, driving wheels etc. This page will show you how to read wheel arrangements if you are new to this hobby.
Some examples of wheel arrangements are:



The wheel arrangement of a locomotive is given by three figures in the case of a normal non-articulated engine. The first figure denotes the number of carrying wheels at the leading end, the second the number of coupled wheels, and the third the number of carrying wheels at the trailing end. 
As indicated in the example diagram (Fig 1.), Garratt and Mallet articulated locomotives are described in a similar manner, except that each unit is dealt with separately, and a plus sign (+) is used between the two units.















Figure 1: Examples of Wheel Arrangements

As you can see on the right (Fig 2), this SAR Class 19D has a wheel arrangement of 4-8-2. The locomotive consists of 4 front bogie wheels, 8 intermediate coupled wheels and 2 trailing bogie wheels.
Obviously, when looking at the locomotive from a side on position, you just need to multiply the different wheel groups by two, to give you the wheel arrangement. This should give you a better understanding of locomotive wheel arrangement.


 All locomotives on the SAR are classified according to their type, and in general, all engines of a particular class are interchangeable, although there are a few exceptions. The class is indicated on each locomotive by small figures, letters, or both, immediately below the running number on the cab number plates. The letter "R" in the classification (e.g. 15AR) indicates that the engine concerned has been reboilered with a standard boiler. The more modern locomotives, which were built with standard boilers, do not carry the letter "R" in their classification.


The illustrated Cab Number Plate on the left, belongs to a Class 25 with a Non-Condensing Tender - hence 25NC. The number denotes the Running / Engine Number.

Here is a Cab Number Plate (Fig. 2) belonging to an SAR Class 19D locomotive. The plates were generally cast in brass and were coated in red paint. The embossed writing was then polished to a high gloss by the drivers and / or firemen.
The standard size of locomotive number plates is 1'8¾" x 1'2½". Most of the number plates were removed from the locomotives by the SAR prior to scrapping, and they are now valuable collectibles (if you can find them), especially when they are found in their original condition.  

Locomotive Frames:

The frame of a locomotive is the foundation to which the various parts are secured. Three types of frame were in use on the SAR, namely, the Plate Frame, the Bar Frame and the Cast Steel Frame.
Fig 1: The Plate Frame
The plate frame consists essentially of two parallel steel plates secured together by a buffer beam at each end, and by various frame stretchers.  Inside each buffer beam is a heavy casting known as the drag box, which carries the drawgear. Many plate frames are fitted near the rear end with a large casting called the bridle casting which in effect widens the frame out so that the lower portion of the firebox can fit inside the frame. Along each side of the frame are large slots, one opposite each coupled wheel, to which are secured the horns, in which the axleboxes for the coupled wheels are fitted. The cylinders and the smokebox saddle are bolted to the frame near the front end, and underneath the saddle is the top Bogie centre casting ­ the pivot on which the bogie can rotate.
Fig 2: The Bar Frame
The bar frame consists of two steel slabs usually from 3 to 4 inches thick, slotted out for lightness. As with the plate frame, a buffer beam and a drag Box casting are fitted at each end, but a bridle casting at the rear end is not often used, as it is possible to sweep a Bar Frame low enough to fit underneath the firebox. Several frame stretchers are fitted. The slots for the coupled wheel axleboxes exist on each side, as on the plate frame, but separate Horns are not necessary as the frame itself is thick enough to form an adequate bearing surface. The cylinders, smokebox saddle, top bogie centre, etc., are bolted to the frame in a manner similar to that used on a plate frame. On the SAR most modern locomotives, and some of the older ones, have bar frames, as there are certain advantages in this type of frame when applied to South African conditions. The most important of these are:


 - that the lower portion of the firebox is easily accessible for washing out and repair.
that bar frames are very rigid, thus largely eliminating any bending sideways on severely curved track.
that springs and compensating gear can be arranged in convenient and accessible positions.
that very few frame bolts and no rivets, which are likely to work loose, are necessary.


The two main disadvantages of the bar frame are its weakness vertically and its weakness near the front end, which can be very simply over­come by fitting deflection plates, commonly known as boiler barrel supports, between the boiler barrel and the top of the frame, and diagonal stays from the smokebox to the front end of the frame. In this way the boiler is used to stiffen the frame.
Fig 3: The Cast Steel Bed
The cast steel locomotive bed was based on a  modification of the bar frame, and was developed in the United States. In general design it is very similar to the bar frame, except that the buffer beams, drag box castings, frame stretchers and cylinders complete with steam chests and smokebox saddle are all cast in one piece with the two main Frame Members.
This reduces the number of bolts required to a very small quantity and has the further advantage, particularly on large locomotives, that the cylinders cannot become loose. There are certain other advantages over the normal bar frame, but as these are mainly finer points of design, they will not be detailed here.


Heat and Combustion:
Heat may be regarded as a form of energy which can to some extent be converted into useful mechanical work. Heat is usually produced by the combustion of fuel, and in the case of the steam locomotive the fuels normally used are coal, oil and wood. On the SAR coal was the only fuel of any importance, and it consists essentially of carbon, a number of complicated chemical compounds of hydrogen and carbon (known as hydrocarbons), and incombustible ash.


When substances combine with oxygen, heat is produced, and when the production of heat is very rapid the reaction is usually called "combustion", and the substance is said to "burn". While coals from different areas vary widely in composition, the general conditions of combustion of coal in a locomotive firebox are the same. The carbon burns on the grate, liberating great heat, and driving off the hydrocarbons as volatile or gassy matter, which burns with luminous flames above the grate. The oxygen necessary to support combustion comes from the air, which consists approximately of one-fifth oxygen and four-fifths other gases, mainly nitrogen, which play no part in combustion.


The result of supplying insufficient air will be to cause smoke, which consists mostly of unburnt hydrocarbons, while if too much air is supplied, some of the heat of the fire will be uselessly absorbed in heating that portion of the air which is additional to that required for complete combustion.


One of the most convenient ways of converting heat into mechanical work, is to use the heat to change water into steam, which in turn can be used in an engine. The water and steam are thus the means of transmitting the heat energy liberated by the combustion of the fuel to the engine, where useful work is produced.