Camelback

A camelback locomotive was designed to burn anthracite waste, a pleantiful and cheap source of fuel which worked best with wide boilers. Wide boilers would have severely restricted driver visibility from the normal cab location at the rear so a cab for the engineer was placed above and astride the boiler. This gave rise to the unusual shape of the camelbacks.

Source
http://en.wikipedia.org/wiki/Camelback_locomotive