There's no step-by-step account for this one, just a summary of what's been done with what. My close-up camera is off for servicing, so these were taken with the standby.
Underframe airbrushed with Railmatch Frame Dirt, then sides with a mix of Frame Dirt and Weathered Black. Top surfaces and cab front and back airbrushed with a darker mix of Frame Dirt and Weathered Black. Wheel centres and coupling rod joints were given a quick waft of Weathered Black.
The tank and cab sides were then brushed vigorously with a wide flat shader while the paint was still damp, to remove enough paint to make those areas look cleaned.
Running plate was given random applications of MIG Productions Rubbel Dust and Dark Mud, the cab roof, chimney and smokebox were given an application of Smoke Black (or is it Black Smoke?) and highlighted metal edges were achieved with AK Interactive Gunmetal pigment. All pigments were applied with a filbert brush.