If the resulting image is flat 2D, no one will be the wiser. But if the image is seen with real depth, the trick is exposed. It will look as if the shadows are painted on a flat surface. The round circle of rivets on the front of the train are one example. You can see the front of the boiler has a round dimple built into it.
Dimple is still visible because of the darker shading below and to the right.
This is the hard part to understand. Besides wrapping a picture of rusting iron, I also wrapped an image that suggests where the circle of rivets should be. This image is used to create false shadows and highlights which in 2-D look like rivets sticking out. But actually, this part of the boiler is flat. You can see that is true if you look past the front of the train obliquely, like the view to the right.