John,
Thanks for sharing the development of this drawing. It's been fascinating to watch the growth. The overall effect of the engine is fantastic.
Could I just add a couple of comments (not criticisms). You've obviously spent hours on the locomotive, then added the trestle bridge. The bridge appears a mere gossamer compared to the heavy engine above it and tends to unbalance the image to me.
You also point out that the front end is the "business" and that's right. How about accentuating it with a bit of receding blur? I'd prefer to see the front of the locomotive in the centre of the span.
Hope you don't mind my comments.