Wiktionary
backhead
n. The wall of a steam locomotive's firebox, located in the cab, where the controls are mounted.
Wikipedia
Backhead
In rail terminology, backhead refers to the wall of a steam locomotive's firebox in the cab of the locomotive. The locomotive's controls are mounted on the backhead.
Usage examples of "backhead".
Now the four big fire boxes added to the light, their doors open to cool the backheads and help draw down the steam pressure.