What Sort of Locomotive is this 4-6-0? - Model Railroader Magazine

Lakeshore Sub

As trains got heavier in the  1870's - 1890's, builders needed bigger engines so they lengthened 4-4-0's with bigger boilers and a third driver set.

It's minimum lengthening, in part, that's behind this.  Think of the 'added' axle as going between the cylinders and the lead driver pair of a 4-4-0, not 'behind', and the structural analogy with a 4-4-0 will become clearer.

Passenger engines benefit from having deep fireboxes, and the 'evolved and perfected' 4-4-0 provides one that can go down nearly to track level.  Naturally this fits between the frames at the back of the locomotive, and for stability you find the driver pairs flanking it fore and aft, with the length of the side rods "longer" to accommodate this.

When you expand to three driver axles, you still have the rear two on opposite ends of the firebox... which is still narrow with deep drop.  But now there is an incentive to shoehorn the two 'leading' driver pairs together and with the new lead pair close to the trailing wheel of the engine truck.

Our modern eyes are used to seeing drivers close together, often so close that brake hangers have to ride outboard, so we 'read' the wheel arrangement as having the rear pair extended, rather than recognizing it as accommodation for stability, deep firebox efficiency, and weight distribution all together.

Note that 4-6-0s with equal spacing and deep fireboxes will have the spacing relatively long (equal in fact to what the firebox requires); 4-6-0s with wide fireboxes carry them above the drivers and have relatively little limitation on wheelbase and relative axle position vs. equalized weight distribution.

The best of both worlds turned out to be wide and deep fireboxes, which necessitated a small carrying wheel at the rear -- in early examples this replaced a rear driver pair in pedestals.  This extended a 4-4-0 to a 4-4-2 and a 4-6-0 (with equal close driver spacing) to a 4-6-2, and that was the first real revolution in modern fast steam.

In my opinion, if you're freelancing a 4-6-0 the added 'size' of the Mantua is only a drawback if it dwarfs the cars you're using.  I would rework the cab to 'scale' when detailing, and just consider the engine to have been built larger.