I certainly don't think a coat of Dull Coat will hurt anything; however, I don't find freight car kits to be needing a coat as they are painted in appropriate colors with flat finishes. If Dull Coat would help when applying weathering powders, this would seem to make some sense. I use Dull Coat to seal weathering powders to my freight/passenger cars. However, I mostly use washes of Acrylic paints to weather, now.
Looking at photos of freight and passenger equipment appropriate for your era should show real weathering jobs from very limited to extreme and depending on condition where the cars are used, new cars can look like they are decades old. Passenger equipment will not be as weathered as freight equipment, because passenger stuff is washed regularly. Freight equipment may never be washed in it's lifetime! To make declarative statements like: "This is how it was!" is O.K. if that's how you feel about it. Many people will probably disagree with you, however!