In addition, their large fireboxes made them particularly suitable for wood burning.

The most practical solution was to add a second trailing axle to spread the increased weight of a larger firebox.

It was the natural development of the 4-4-0 the additional trailing truck not only supporting a larger firebox but improving the riding.

A larger firebox similar to the ones on the 2-6-6-6 locomotives built by Lima would have been fitted, allowing for greater power at speed.

In 1909 and 1910, the original locomotives also received this larger firebox.

This would have allowed an even larger firebox than the 4-8-4.

The large fireboxes of these locomotives were made obsolete by better boiler design.

Wootten determined that a large, wide firebox would work best.

Besides being more expensive, coal required a larger firebox in which to burn.

The changes were fundamentally confined to a longer wheelbase to permit fitting of a larger firebox.