Between April 1912 and 1922, the South African Railways placed all together 46 Class 12 steam locomotives with a 4-8-2 type wheel arrangement in goods train service.

[1][2][3][5] In June 1921, Beyer, Peacock & Company (BP) delivered the final twenty Class 12 locomotives, built in two batches of ten and numbered in the range from 1859 to 1878.

[1][2][8] When the Class 12 was introduced, it was the largest non-articulated engine in South Africa and possibly in the world on railways of less than 4 feet 8+1⁄2 inches (1,435 millimetres) gauge.

[2][5] The NBL-built locomotives were very similar in appearance to the Class 3B, with running boards which curved down below the cab at the rear and down to meet the buffer beam at the front.

With the earlier cabs, many drivers wore leather guards on their left legs to protect them from the heat when the fire door was open.

The BP-built locomotives had larger and more spacious cabs to provide increased comfort for the enginemen, and running boards which ran straight through to the rear buffer beams.

[2][3] During the 1930s, many serving locomotives were reboilered with a standard boiler type designed by then CME A.G. Watson as part of his standardisation policy.

In the case of the NBL-built locomotives, the reboilered engines were also fitted with running boards which continued straight through underneath their cabs.

A handrail was provided, passing round the back of the footplate without impeding the stoker's access to the coal bunker, an arrangement which was subsequently adopted as standard practice.

Even though they were subsequently allocated to dock shunting and yard work in the New Brighton marshalling yards, they still assisted with mainline work when required and, in spite of their small 51 inches (1,295 millimetres) diameter coupled wheels, were comfortable at 60 miles per hour (97 kilometres per hour) when employed in tandem with the larger Class 15F.

Although they had been drafted in for these old-age duties, the phenomenal growth in traffic during the 1960s had the result that the Class 12R were also frequently used for road jobs on the Kroonstad as well as the Bethlehem-Harrismith mainlines.