Wolfgang, great idea about using the felt tip pens. I've often wondered about how well they would do but it seems that they look pretty good. Certainly a lot easier to control than paint.
Chas, my son is also a structural and is always critiquing things like my structure kits and bridges...in a nice way, of course, just letting me know how things would look in real life. He's seen a few of my more heavily weathered cars and told me no railroad would run something that looked that bad. He was shocked when I showed him the prototype pics of the cars. I told him railroads pretty much run 'em now until the wheels fall off.