The main advantages of the Garratt are high tractive effort, large boiler size and the ability to negotiate sharply curved track with ease. If you think of the minimum radius needed for twelve or sixteen coupled wheels you can see at once where the advantage of an articulated loco lies. Mallets achieved much the same but did not have the flexibility of the Garratt - the combination of two power bogies with a boiler slung between also allowed much bigger boiler / firebox combinations than would otherwise be possible, particularly on 3'6" lines or narrower. Apart from the first couple, Garratts were also built as simple engines and thus avoided the complication of compounding (as with many Mallets) which meant they were better suited to the rough and tumble of colonial lines.
I forgot Doug's illustration - good example, 4-8-2 + 2-8-4 - sixteen coupled wheels!
The other innovation developed for the Garratts was the rotating, self-trimming coal hopper to carry more coal than a fireman could ever reach from an ordinary shovelling plate. Did they try under-fed mechanical stokers?
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