Engines to pull them, engines to bring back the empties to reload with cars. Much more profitable to pull tank cars, I reckon. Look for shipping surcharges to be added to the new vehicle price.
At the railroad that I work for, we’re pulling all of our older locmotives out of long term storage, and putting them back into service for this very reason. Carloading are through the roof right now, and not enough power to pull them. A lot of those return to service locomotives are also going to the Canadian railroads as horsepower payback. Also, tank car manufacturing is running at capacity right now.