Path, it's not the economy I was questioning, but the end-to-end speed.
My brief sojourn into the shipping world, I never thought of trains as being a speedy way to ship. It always seemed the time to get the trailer to the yard, get it loaded, then scheduled for a train, switched and finally reverse all that at the destination added several days to the schedule. If the shipper/buyer can wait then that's okay, but for the bulk of UPS/FedEx stuff I would imagine it might be a tad slow.
Just FYI, my experience was with Norfolk Southern, but that was 20 years ago. I know things have changed. Are they that organized that it's makes sense with a route as short as TX to AZ?