Very cool, Nice hairspray technique on that!
The rust looks a little reddish to me, and a little too consistent, tone-wise. Try adding some bright orange to it for fresh rust in places like the tracks, which would have been abraded the last time it moved, and then freshly re-rusted in some brighter tones. Pigments would work great there.
Some light streaking with maybe some white oils on the side would give it some flair, and lastly, put some bird droppings on the top of it. :)