I brought up something like this idea 2-3 years ago when the C&TS had zero motive power and was promptly thrown to the lions!
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I don't recall any Garratts in SA being other than normal with the running gear and wheels outside the frame(inside frame if you want to classify it like D&RGW).
When I was there in 1977 and 1984 there was primarily one class left, the GMAM 4-8-2 + 2-8-4.
I never got over to see the narrow gauge ones(they were 2 foot), but I think they were inside frame as well.
Zimbabwe still has some Garratts running today, and I believe have 3 different classes serviceable. They made quite a stir in the railfan world last fall when they put 2 or 3 Garratts back in commuter service in Bulawayo. Each engine did something like 2 trains each in the morning and 2 each in the afternoon on two separate lines. They still use Garratts in shunting(sorry switching)duties around Bulawayo most days. My brother was there in 1990 when steam was still on regular longer trains.
Greg Scholl