The railroads DO NOT OWN nor DO THEY PURCHASE tank cars for transporting liquids. The cars are owned by the builders or major lessors and leased to various shippers. Next time you watch a train go by look at the tank cars. You may notice that ALL OF THEM have an X at the end of their reporting marks. TILX, SHPX, UTLX, etc, etc. Any time you have a car with an X you have a non railroad owned car.
They for the most part don't even own coal cars anymore. They are owned by the power companies or other transport leassors also. Not owning cars means they have less money tied up in cars and they can do more with that cash in other places, like the right of way. Don't have to buy them, or fix them, they just run them.