Well, I made a sketch in between seeing customers (I won't tell anyone if you don't) and this is what I have planned:

I am tossing up whether or not have the marquee/tent on the top or room for a gun emplacement... can I put both? An interchangable thingymabob?
I also drew some designs for a boiler:

The intake door is presumed to be poking into the front cabin, where the crew toil away to get the beast moving. Does this make sense?
Some guidance here would be very much appreciated!