Here's an "old" Atlas gondola that I modified and weathered several years ago. It was originally decorated for the Norfolk and Western. I repainted and re-lettered it for the Jersey Central, replaced the old Atlas plastic trucks and couplers with Weaver die-cast ones, added brake hoses and a few wire grab irons...and of course weathered it. The awesome "muck" load was made for me by fellow Forum member "Rail" (aka Don K.) @rail.
Unfortunately, it was one of my very first attempts at repainting a freight car, before I knew how to remove lettering, so you can see the N&W lettering under the new paint job if you get up close...but it looks great from 4' away! 
The new CNJ lettering was a set of dry transfers from CDS Lettering. Wish they were still around!
Weathering was done with acrylics, oils, chalks and powders. This was before I discovered pan pastels. White chalk was used to represent the fading/running of the white lettering.
A few more photos below.
