Since the story was historical fiction, I relied heavily on photographs to construct my illustrations. When I had roughed out this composition, using xeroxes of photos and my own very rough figure sketches, I showed it to my dad, Aldren A Watson, the well-known illustrator who was my professional mentor until his death. Dad looked at my compostion for several minutes, clearly uncomfortable about SOMETHING. Finally he said, "You've got an American steam engine here, not a European steam engine."
As it happened, my father was not only a terrific artist, but an expert on old trains. (And just a note: I learned that American steam engines had cowcatchers at the front end; European engines had bumpers.) He sent me sketches, photos, and books of European steam engines, as well as historical information about the manufacturers of steam engines, indicating which kind of engine was likely to have been in Warsaw in 1940. Thank goodness he caught this mistake---I'm not sure if anyone else would have seen it until it was too late.
I began to revise my sketch. It was hard to find photos clear enough for me to see all the details of wheels, drive shafts, and other mechanical parts. "Just add a lot of steam" my Dad said. We artists know about "adding steam". Sometimes that's a cop-out; other times it's a practical solution. As my son always said about film and the stage (and equally applicable to illustration): "We're creating an illusion of reality, not reality itself."
On a separate transparent layer I next worked out very carefully the arrangement of the clouds and steam behind the engine.The separate layer avoids messing up any drawing on other sections of the compositions.
Next came the figures of the pursuers...
...and of the pursued.
I combined all these layers into my master sketch. I carry out this process with transparent vellum, but the process is exactly the same if one is doing it on a computer.
And voila (after 4 days of painting and inking)---the finished illustration.