Taylor,
The South African 25 and 25NC class 4-8-4's were about as big and modern as non-articulated narrow gauge ever got. Cape gauge is only 6" wider than the D&RGW, but these engines developed 3000hp, had roller bearing rods and could run at 75 mph all day long. This isn't your father's narrow gauge, as they say!
BTW, the 25 class were built as condensors for use in the Karoo desert, while the 25NC's were built non condensing for use elsewhere. Most of the 25's had their condensing equipment removed, but you can tell them by the huge "sausage" tenders. The 25NC's have been compared to 5/8 scale New York Central Niagaras!
Here's a shot of an original class 25.