The Walthers 86 ft HC box cars are sweet. This will relegate all but a few of the blue box units to stationary prop status in the yard. Mostly a light weathering of an oil wash for general grime and dust pigment along the bottom of the sides and on the ends for good measure. Just a little hint of surface rust on a few roof seams.
A little more dirt on the car by applying pigment over the oil wash.
Based on photos of these GTW cars on the NS at Bluefield, WV, I created a fade by airbrushing a white paint wash to the complete car. This was followed by soot pigment for additional grime along the sides and ends.