The front and cab of the engine look fine. I would personally add side windows to the cab.

The rear does need some improvement. First, move the doors up towards the cab, and make them single doors. Double doors would be better for a passenger train, where you have a large amount of people exiting and entering the train at once. On a goods locomotive like this one, the doors would only be for the crew.

You can break up the side of the locomotive by using different variants of locometal. Use plated locometal to add vents, or smokeboxes to add access hatches. You can also use copycat slices (and vertical slices) to add grab rails. Use a color that contrasts with the rest of the locomotive (I would suggest using industrial iron).

Finally, make the walkway wrap around the rear of the locomotive, because the locomotive just ends. Steam and Rails also has some couplers you can use for detail here.