If they're full, they'd have seals on top and bottom(if applicable). Those seals will have a commodity tag attached. If hazardous, it will have DOT placards. Rail car numbers, most of which start with letters(UTLX, DOWX, etc.), tell who owns the rail car. I know I can look on a program at work to find this information but not sure if there's a public website to get it. Cars without placards and no commodity tags could actually be recently cleaned and empty. With companies slowing down processes, because lack of transportation, and customers not purchasing, these cars could very well be empty and being stored until demand comes back up. Not saying something nefarious isn't at play. Just saying this is typically how it works.
Or they're being transported without any markings so people can't figure out what's in them.