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