I believe brass can be made into a much more realistic model simply because the shell or body has a prototypical thickness. This leads to two advantages when considering fidelity to the prototype:
1) Window frames, box car doors, box car sides, hopper car walls, flat car decks, etc can be made with a prototypical thickness. No need to have 6” thick windshield frames, passenger car sides, caboose windows, caboose platforms, etc.
2) Diesel and carbody sides, boilers, etc are not dead flat, because they are formed, rather than cast from a precision mold. Sometimes they are soldered together from many pieces. This naturally gives a variation in the surface flatness that is just like the real thing. While subtle, to my eye this enhances the overall sense of realism.
Yes, you can add brass details such as grab irons, ladders, etched screens, etc to a plastic or die cast model. And it will look pretty good. But for absolute realism, I can’t get around the fact that the walls are too thick and too perfect.
And yes, I have plenty of plastic diesels and rolling stock:
