Hey Yuze Gize,

 

In my opinion it doesn't have to take a badly fired steam engine to have the amount of crud, be it steam soot or spent diesel smoke to accumulate on the top of a load of lumber like this. The scene posted ~appears~ to be Eugene and the origin of the load might likely be one of SP's coastal lines like Tillamook, and with the amount of precipitation and tunnels that the train would have to traverse this could be a two day occurrence. The precipitation embeds the crud into the top layers of lumber on it way to Brooklyn then Eugene and then points eat over the tunnels on the Willamette past or over Mount Ashland... Next Roseville and choose your poison boys the Sierra Nevada's, the Great Salt Lake or the Tehachapi's.  It get cruddy, it stays cruddy and if you are a young man of  twenty something and you have to strip these cars and prep for the lumber lines in the yard flipping the top boards and re-banding the units make you a pretty grungy guy when you head home. SO believe it this is weathering 101.

 

I will agree with Armand, weathering is a matter of choice but realism is realism and as they say model from photos...

 

Greg Martin