sorry, I thought you were referring to the new route. But to answer your question, the auto racks in Heavy Haul were originally full during the beta but was taken out when it was released. I would imagine they did it for performance reasons, as far as the other routes I’m not sure, but I know the new one will have full wagons.
you can download a mod somewhere that fills up the autoracks on heavy haul see this post https://forums.dovetailgames.com/threads/why-are-freight-cars-all-empty.20574/
Just for the record: In the new Oakville route, some (perhaps all) centerbeams are loaded. My guess is that most freight cars are empty to keep their loads from adding to TSW's already hefty graphics burden.