Gavin I would suggest that Garratts are more evenly weight distributed than most normal articulateds(if normal is the correct word).
Garratts have a set of drivers under the water tank, and the other set under the coal bunker, with the boiler slung in the middle of both drivers. The idea of Garratts was to spread the weight out to allow them to function on light rail and still get adhesion.
In South Africa we noted a lot of engines slipping, not necessarily Garratts, but straight engines as well, and we noted a lack of sanders seemed to be the main problem. Do they have sanders on 25nc's for example?
Back to the articulateds, Coker is right. I have seen Challenger 3985, and lots of times Norfolk and Western 2-6-6-4 in action, and the front driver set is normally the one that slips. I have a long video sequence of 1218 climbing into east Bluefield yard on a wet day and the exhaust is in and out of sync and slipping on the front driver. There is more boiler weight with teh firebox and so forth at the rear, plus the gradient(upgrade) puts more weight on the rear.
Greg Scholl