A dark mix of Railmatch Weathered Black and Frame Dirt has been airbrushed over the top of the boiler, the cab roof and the driving wheel centres. This is to represent the soot present on the former two and the oil on the latter. The photograph that I have been using as a reference shows a much more marked contrast between the wheel rims and the wheel centres, but it does not look quite right to my eye so I have reduced the contrast by using a slightly darker shade of brown.
Black Smoke pigment has been used on the smokebox and its door, with a very light application of the same to the front corner and edge of the tanks to show where water continually runs if the filling process means the tanks overflow.
Not far to go now. Thanks for sticking with it!