On a lighter note - it's Steampunk. And I believe the place it falls in the tech tree, at least in our scenario, is after Automata. So why would there need to be gun crews at all? Furthermore, why would one assume it is using steam, strictly defined?
Since publication of the details is restricted under the Official Secrets Act of 1889 I will just say that the motive force is provided by an engine encorporating the key technologies of the Maestrofoon, W. G. Armstrong, & Du Tremblay designs. Such a combination reduces engine size, eliminates the need for large boilers or capacious fuel reserves, deals with the exhaust profile issue, and provides auxilliary power for electrickal illumination and fire control Babbage engines. And, it must be pointed out, the previous correspondent has completely misunderstood the scale of the vehicle - which is more comparable to the Wells device under construction than to a rude farmer's tractor. Sufficient room for the small crew required.