I know there were at least two South African locomotives included with Trainz 2004 by default (a 34 Class diesel electric and a 6E1 class electric). If you register your copy of Trainz you should be able to find them on the Download Station (DLS) without too much trouble.
Note that these have standard gauge bogies though instead of 42" gauge ones, due to them being old content from before narrow gauge track became common in Trainz (you can run them on narrow gauge as is though without any trouble, as the software doesn't differentiate between track gauges - every train and track is the same gauge as far as it's concerned).
Additionally, I know Austin36Hockey made a basic model of a 31 class Garratt, but as far as I'm aware that was only available on a third party website which I cannot remember the link for anymore (it's not on the DLS) - in any event it was Trainz 1.3 content and has a few errors which mean it would most likely not work in newer versions of Trainz without fixing (other than 2010's Compatibility Mode).
Finally, speaking of Garratts, althoughit's not 3'6" the 2 foot gauge SAR-SAS NGG16 class Garratt as preserved by the Festinog railway in Wales is available for download on Trainzone (
http://www.trainzone.co.nz) in several different liveries.
I'm not sure there is any specific South African rollingstock available however.
