There are a number of references, none yet quite rigorous, which point out the superiority of a proper deep firebox with circulators over something with a wide grate ‘over the drivers’. This is further enhanced if active circulation is provided in the water legs (a la Cunningham circulator, which draws from a downcoming region in the convection section of the boiler, and uses a jet pump to distribute it through nozzles in the outer wrapper above the mud ring).
The deep firebox implies greater mass on the trailing truck, both from the additional metal structure and weight of water. Note that on C&O, which had dramatically high axle-load capacity, the Alleghenies have a six-wheel trailing truck.
Rear-end stability on a deep-firebox engine of suitable capacity is almost incomparably better than a Challenger. Look at the Bissel formula that keeps the truck wheelbase ‘normal to the railhead’ in curves, then extend the truck out so rear bearing and steering forces are as far outboard and to the rear of the chassis as possible, and angle the restoring-force devices (usually rockers or segments of gears) to match the swing radius at the rear.
In a pinch, you could use the dodge that was introduced in the ‘intermediate’ Berkshire trailing truck frame, when a long frame pivoted at the original ‘articulation’ point was used as a Delta-style trailer. This was treated dynamically as a long 2-wheel Delta trailer, with the leading axle only weight-bearing – it could float laterally on a pair of hardened-steel rollers independently of truck-frame angularity. Any of the subsequent schemes of lateral-motion compliance could be used on such an axle if desired.
The only American engines with a deep firebox and divided drive that used a four-wheel lead truck were the PRR Qs, and those did not have the ‘compound-pendulum’ guiding concerns of a Mallet-style chassis. All the six- and eight-coupled simple articulateds with