My two cents:

1. Richard Hendrickson's article "Vintage Dating Freight Cars" in the December 1995 issue of Railmodel Journal is required reading, not for specific weathering techniques, but the importance of context in capturing the overall scene on your layout.  The article is available online at http://magazine.trainlife.com/rmj_1995_12/  .  (Go to page 32.)

In Richard's words,

"Freight car fleets keep evolving. Older cars are retired, modernized, rebuiIt and/or renumbered. New cars are delivered. New paint and lettering schemes are introduced and gradually replace earlier ones. The process is end­less, not only on the prototype railroad you're modeling but on the other lines whose cars turn up on your railroad in interchange service. Making your freight car models realistic, then, isn't enough; not only should each car repre­sent its prototype accurately but all your cars should look the way their pro­totypes did at the same moment in time."

Just as having a fleet of brand new cars is unrealistic, having an entire fleet weathered to the brink of extinction is also unrealistic.  (The most notorious example of this is a heavily weathered Athearn Santa Fe "Shock Control" boxcar that turned up regularly in photos of Sellios' Franklin & South Manchester.) 

2.  Weathering from memory is tough as what you initially think is credible weathering turns out to be atypical or odd.  (An example is the Ertl boxcars and gon "flood recovery" weathering.)  Keep color photos handy for inspiration - an excellent source is the Library of Congress Farm Security Administration/Office of War Information Colllection.  These are often misidentified on these lists as from "Shorpy" or "Collection of Shorpy", but the reality is these are in the public domain and available online, and the collection is far more extensive than what is posted on that blog.

https://www.loc.gov/collections/fsa-owi-color-photographs/about-this-collection/