Covered hoppers were free runners in theory, but that was tempered by several factors:
* private-owner cars in dedicated lease service might have run in particular, restricted routes
* the limited commodities carried by covered hoppers and the availability of local sources for those goods
* cicra 1950 there were only about 15,000 covered hoppers in service, versus 750,000 or so box cars, so they would have been a fairly rare sight to begin with. Statistically, you should have ten tank cars and nine reefers per covered hopper.