The railroad is almost in a win-win situation. PSR will either be great for the shareholders or it will be a disaster and no one knows for sure which it will be now. If its successful, then well good for you UP, you'll make billions with only 70% of your previous labor cost and a huge reduction in capital.
But... if PSR fails, you're going to have to hire people back and try to restore what you were doing before that came along. Of course the new employees will be coming in making much less than those who left in 2018-19, and will have less benefits. You'll have to earn back business, but companies will be happy to sign up again to ship their good providing they cost less than trucks.
I think @XzlnUkw-dsd really hit the nail on the head.