The outer firebox had a Belpaire cover, a deep firebox that extended below the axles and a large smokebox.

The 2-8-2 layout of the X class allowed a wide, deep firebox and large, free steaming boiler.

Also, the Kitson-Meyer variant with the trucks spread apart to allow a deeper firebox between them.

The most significant change was a deeper firebox with shallower sloping fire grate.

One advantage of the type over its predecessor 4-4-0 American type was that the trailing wheels allowed a larger and deeper firebox to be placed behind the driving wheels.

These locomotives had a deeper firebox, steel cab and weighed an extra nine tonnes.

Although all were constructed to the same basic design, the locomotives that were outshopped from 1884 had slightly larger side water tanks and deeper fireboxes, therefore increasing efficiency.

The driving wheels were also located further forward to accommodate the deeper firebox and as a result the leading bogie was replaced by a single axle bissel truck.

The trailing truck allows a larger, deeper firebox than that of a 2-10-0.

This casting extended out on either side and had wider spaced frames attached to it, which allowed a wider and deeper firebox.