Cab interiors on preserved engines are lovely shiny places. Because the footplate crew are volunteers, a bit of elbow grease and Brasso isn't a problem. Indeed, they are so chuffed to be there and allowed to be in charge of a real life locomotive, that they want to show their steed off to the public and make sure it looks it's best.
The cab of 828 is a good case in point. You can see your face, or any other part of your body you care to examine, in the pipework. The cab sides are clean and you can easily see that they are painted with a woodgrain effect. Even the backhead has had attention.
In real life, things weren't so tidy. Crews might have been a little house-proud when they were assigned to a single engine but once you were rota'd on to anything that moved, the impetus to spruce it up wasn't really there. The next guy on the footplate might benefit but you could spend your life polishing and by the time you saw your engine again, the work would have been undone.
On the C15, I wanted to show a working engine. It's not decrepit but the cab is a dirty, dusty place. The sort of effect you'd get by shovelling a few tons of coal from one end of your living room to the other.
After a little discussion on a forum, it was decided the the cab sides would probably be painted beige/cream above waist height. Left on it's own, this would be too bright but a wash or two of dirty black toned it down and gave a pleasantly uneven finish. Once the paint had dried but not hardened, I brushed several colours of weathering powders and some black powder paint around to make things even dirtier and properly gritty. It's not clean but not horribly dirty either. Hopefully more real engine than museum piece.