In 1927, the SAR began to convert the couplers of its Cape Gauge rolling stock from the Johnston link-and-pin coupling system which had been in use since the establishment of the Cape Government Railways in 1873, to AAR knuckle couplers.
Judging from contemporary photographs as well as the official SAR Locomotive Diagram Book and the dimensional locomotive drawings as published by Holland, which were for the most part based on the original as-delivered and unmodified loco­motives, the Class 18 locomotives were delivered new with knuckle couplers fitted, as were the Classes GCA, GF, HF and U which also entered service in 1927.

Conversion of all rolling stock would take several years and both coupler types could still be seen on rolling stock into the late 1950s.
During the transition period, knuckle couplers on locomotives had a horizontal gap and a vertical hole in the knuckle itself to accommodate a link and a pin respectively.
This enabled them to couple to vehicles which were still equipped with the older Johnston couplers.

Knuckle couplers had first been used in South Africa more than two decades earlier.
The Central South African Railways (CSAR) introduced Gould knuckle couplers on the rolling stock of its Limited Express and Imperial Mail passenger trains in 1904.
The Limited Express operated between Pretoria and Johannesburg while the Imperial Mail operated between Pretoria and Cape Town.
These knuckle-couplers also had split knuckles to accommodate coupling to the old Johnston couplers with a link and pin, since the CSAR retained the old couplers on all their locomotives to keep them compatible with their own goods and older passenger rolling stock as well as with that from the other railways it connected with.
In 1927, the SAR began to convert the couplers of its Cape Gauge rolling stock from the Johnston link-and-pin coupling system which had been in use since the establishment of the Cape Government Railways in 1873, to AAR knuckle couplers. Judging from contemporary photographs as well as the official SAR Locomotive Diagram Book and the dimensional locomotive drawings as published by Holland, which were for the most part based on the original as-delivered and unmodified loco­motives, the Class 18 locomotives were delivered new with knuckle couplers fitted, as were the Classes GCA, GF, HF and U which also entered service in 1927.
Conversion of all rolling stock would take several years and both coupler types could still be seen on rolling stock into the late 1950s. During the transition period, knuckle couplers on locomotives had a horizontal gap and a vertical hole in the knuckle itself to accommodate a link and a pin respectively. This enabled them to couple to vehicles which were still equipped with the older Johnston couplers.
Knuckle couplers had first been used in South Africa more than two decades earlier. The Central South African Railways (CSAR) introduced Gould knuckle couplers on the rolling stock of its Limited Express and Imperial Mail passenger trains in 1904. The Limited Express operated between Pretoria and Johannesburg while the Imperial Mail operated between Pretoria and Cape Town. These knuckle-couplers also had split knuckles to accommodate coupling to the old Johnston couplers with a link and pin, since the CSAR retained the old couplers on all their locomotives to keep them compatible with their own goods and older passenger rolling stock as well as with that from the other railways it connected with.
South African Class 18 2-10-2