Hi Fred,
The HOn3 Blackstone cars have the same problem. Are the Bachmann roofs black or green? Either way, what I've done is mix up some washes both a little lighter and a little darker than the base roof, and then streaked them in the way water would run off the roof.
I did the same with the Silica bunkhouse (a re-used combine body):
The effect is pretty subtle, but I think that's what you want as the passenger fleet was pretty-well maintained even in the hard times.
My coal cars, for instance, are much more heavily weathered:
Cheers,
Jeff.