I'll look up the MR and report back in more detail. But here is the basic technique as I recall offhand:
Airbrush part of the model with ModelFlex Rail Brown, and as it starts to dry wipe off most of the paint with a Q-Tip dipped in Badger Airbrush cleaner, using downward vertical strokes. The remaining paint accumulates around rivets/seams and looks like rain-washed grime. According to the author, you should work on a small part of the model at a time, so that the paint doesn't get too dry & become too hard to work.
The author does HO but I was wondering if this would scale to N reasonably well. Seems you'd have to be careful to avoid overdoing the airbrush cleaner, lest the underlying pain become damaged.