Overall, a nice job. Skipping over ther previously mentioned items, I'd like to bring up a few other things to think about.
First, it looks like you have a coat of mud on the ride surfaces of the road wheels. Those areas would be kept cleaner by the constant contact with the tracks. Following the same thought track, there would be some indication on the tracks where the wheels are running.
Second, the rust color on the exhaust covers seems a bit too uniform. Some tonal variations may add interest to them. The same could be said for the logs (a light dry brushing to bring out the texture), and the tools (though you really only show the pick, I imagine the other tools are finished similarly).
Hope these thoughts help you out.