I think the idea that a unit train requiers only one waybill is a good point. There may be a couple of others that define a modern unit train. One would be that the cars stay together, and two that they are in a dedicated service. For example above, the same two tank cars always back and forth between the same loading point and the same destination.
In reality is there a definition, however? Do the railroads have a category called "unit train"
