This begs the question, how much does it cost to lay road than to lay rail? If it's going to be exclusively autonomous, the more narrow confines of the rail should be well suited.
To paraphrase Churchill, trucking is the worst form of transportation, besides all the others. If you can put freight on a train, you do. It's slower, point-to-point, and less good a less-than-trainload freight than trucks are.