WHR Garratts weigh 60 tons (around 130,000lb) and run with no trouble on 60lb/yard track, mainly on steel sleepers (ties) which 'dig in' on sharp curves and hold the track position better than wood.
The individual axle load is very low (average under 7 tons (13,000 lb) per axle) and speeds do not get above 25mph and are mostly in the 15 to 20 mph range.
However, rebuild of a 'Kalahari' Mikado is well advanced -- a new tender body with cab shelter for reverse running is a feature -- and it will be interesting to see how this loco performs when finished.
Bob Gartside