Photos of the Park Royal railcar are surprisingly difficult to find. Colour pictures appear to have been taken on film based on unicorn skin. The only ones I’ve found, in "Scottish Region Colour Album No 1" by George C. O'Hara, are gloomy and really only show the vehicle end on.
Only a single picture gives me any indication of the state the sides got in after a little running. Basically the underframe is filthy. Really filthy. This contrasts with pretty clean bodywork. At a guess these things were looked after by the shed staff who cleaned the shiny bits and ignored the rest. I’ve never entirely understood this, surely you want the mechanical bits kept clean as well so the dirt doesn’t stop them working ?
Anyway, in mode form my solution has been to paint everything below the body with Precision Weathered black. This was dry-brushed with some rust and gunmetal. Then the mechanical bits are airbrushed with a few dilute coats of track colour. Finally I brought the body and chassis together and wafted more very dilute brown along them both. The look I’m going for is a railcar that is in service and has been cleaned but not recently.