...the answer is the later!
I spent this morning fiddling around with the Steam Tank. Lowered the deck, while keeping the sides - looks more ship like - added a boiler bulkhead, and then found and added some air vents for the crew compartment...
"Cool," thought I, "now to print it...". Hmmm, with the air vents the paper model suddenly ballooned out to 23 pages with lots of little fiddly bits. Far too complicated.
So I've decided to print the model sans air vents, and hope to find someone to make some air vents (and funnels, if I can) for me. I hope. Those vents look way cool!