few things..
comparing the ref to drawing-
get in the habit of squinting your eyes...and do so now, and see that you have much more shadowing happening (within circled areas) than you are representing. The result of not emphasizing the darker values is that this will then flatten the 3 dimensional depth perspective and illusion of your work. Better to over-exaggerate this value contrast, than understate it...
Secondly...your wheel is not round. It has three distinct flat edges that would cause this train car to bounce...
When creating the illusion of depth, do not think across your paper, or up and down. Think of it in visual terms more like a cube...
In this simple illustration of a scene...the visual balance must also include what goes back in space-
Take this reference of a place I like to paint at...
here is how you must think then to execute such a drawing/painting as it should be...
...the point being then, you need to see that the deepening of the values representing the planes going away from the eye will then be like each plane of the landscape above going back. Fail to represent each plane with adequate enough value rendering, and you flatten the illusion...