A new design of main line 4-8-2 locomotive was introduced by CME W.A.J. Day, which first saw service in 1938.

The original design called for 1676mm (5' 6") diameter driving wheels. However this would have meant a new design of boiler to accommodate the extra length. Because of the deterioration of the world situation, it became imperative to obtain these locomotives as quickly as possible. As there was no time to design a new boiler, the standard 3B was adopted, albeit with an increased pressure of 1551kPa (225 psi), the highest yet used on SAR. The coupled wheel diameter was reduced to 1600 mm (5' 3") and an extra long smokebox, to compensate for the increased length of the engine was provided. The last of the 136 engines was delivered in August 1939, one month before the outbreak of the Second World War.

In general appearance these engines were similar to the Class 15F although the boiler is set lower than on the 15F's and a slightly longer chimney is used. These engines were intended for working over the Karoo section where water supplies were erratic, so very large tenders with high water capacity of 43190 litres (9500 gallons) were used.

When the 23's were withdrawn many of these tenders were fitted to Class 15F locomotives and continue in service today.

At the time that these engines were ordered, streamlining was in fashion in the United States and Europe and it was proposed to adopt it on some of the 23's. However, investigation showed that the increase in weight and cost could not be justified for the relatively slow South African line speeds and the idea was dropped.

The 23's were placed in service on the Cape Town - Touws River - Beaufort West and De Aar - Klerksdorp sections where they formed the bulk of the motive power for some years. When they were displaced by the introduction of the 25 and 25NC classes as well as by electrification many of the class were transferred to the Orange Free State. In later years they worked from Kroonstad to Klerksdorp, Bethlehem to Bloemfontein and from Bloemfontein to Bethlehem, Noupoort and Kimberley. Metal fatigue in the bar frames of this class caused their relatively early demise and they were all withdrawn from service by the late 1970's. Fortunately, one locomotive, No. 3300 is retained at Bloemfontein for special trains. She was without doubt, one of the cleanest locomotives in the country and in the hands of an experienced driver is one of the most thrilling locomotives to travel behind on SAR.